My responses to: 50 Questions (mostly) about Ponies and Equestria!.
EDIT: I was surprised by this, I actually have more headcanon than I thought I did.
1. How old are Celestia and Luna?
Celestia and Luna came along sometime between the founding of Equestria and the reign of Discord.
2. How old is Cadance?
Not that much older than Twilight, equivalent to 5-8 human years.
3. Were Celestia and Luna always alicorns, or did they ascend?
Prior to Magical Mystery Cure I would have said they were always alicorns, but now I lean toward the idea they ascended.
My personal headcanon is that they got their cutie marks/ascended while wrestling control of the sun/moon back from Discord.
4. Are Cadance and Twilight immortal? 5. Have there been other alicorns in the past?
Cadance's existence implies alicorns are more common that we once thought. If so, they must not be immortal or Equestria would be overrun with them by now.
6. How much authority do Celestia and Luna have in Equestria?
Pretty much absolute power, perhaps limited by a few traditions which they could technically break but would face a lot of pushback for doing so.
7. Does Shining Armor rule The Crystal Empire along side of Cadance?
Shining shares rule of the Empire with Cadance, though not any of her alicorn duties.
8. Other than Twilight, Luna, and Cadance, what relationships have been important to Celestia in her lifetime (students, close friends, lovers, family)?
There was Sunset Shimmer, of course. I think Celestia has had other students as well. I'm not sure Celestia's ever allowed herself a true friend other than Luna.
Ponies and Equestrian Culture:
9. Are there still cultural differences between earth ponies, pegasi, and unicorns, or is the culture homogeneous by the time shown in canon? Are there cultural stereotypes (positive or negative)?
Although Equestria has become much more homogenous since it's founding, there are still a few of each tribe's traditions that linger on.
Cultural stereotypes aren't common in most parts of Equestria, the exception being places like Cloudsdale where only one tribe is normally present.
10. Are there foods or items native to Cloudsdale that are uncommon on the ground? Are there foods or items on the ground that are uncommon in Cloudsdale?
Weather-related items like rainbow juice are uncommon on the ground.
If Cloudsdale can produce any kind of food it must be uncommon or else the pegasi wouldn't have been starving in the Hearth's Warming Eve tale.
11. Can all unicorns learn all spells, given the time and effort spent practicing them, or is magical talent usually limited in some way?
Most unicorns find it difficult to learn magic not related to their special talent, but it is possible with enough study.
12. Earth pony magic: Does it exist? What is it?
Yes it exists. Since it's never been explicitly shown, it must be very subtle and unobtrusive. I think earth ponies' magic is if anything even more individualistic than unicorn magic - one earth pony might be incredibly strong, one might fantastic at growing plants, and one might be able to cleave giant rocks with their hooves.
14. How much formal schooling is an average, middle class pony expected to complete?
Equestria has a public education system which provides its citizens with the equivalent of a high school diploma. Since all of the educators have a special talent for teaching, this probably doesn't actually take twelve years to accomplish. Beyond that point a pony's studies are guided by their special talent.
15. What’s the average lifespan for a pony? At what age is a pony expected to be independent of their parents?
Lifespan could be anywhere from normal human to hundreds of years.
Ponies can become independent a year or two after earning their cutie mark - though Equestrian society doesn't make a big deal out of ponies leaving home.
16. Are there roughly an equal number of male and female ponies?
17. How informed are most ponies about things that happen in other parts of Equestria? What about other parts of the world?
Newspapers are dominated by local news. Coverage of events in other cities only applies to major happenings (disaster, royal wedding, political or economic upheaval) and events beyond Equestria's borders are largely limited to matters of war and diplomacy.
The Main Characters:
18. How old are the Mane Six? Spike? The CMC?
The Mane Six are young adults, the CMC are children on the cusp of adolescence, Spike is an adolescent.
19. Did Fluttershy remain on the ground after getting her cutie mark? Were she and Rainbow Dash friends all along, or did they drift apart for a while?
The flight camp instructors would eventually have found her and returned her to the camp. I imagine she moved groundside as soon as she could - probably within a year.
I'm not sure Fluttershy and Rainbow were all that close at flight camp, but by the time Twilight came to Ponyville they had definitely drifted apart.
20. Rarity and Applejack both seem to have grown up in Ponyville. What were their interactions like before the show?
Polite but distant. Rarity would have regularly interacted with AJ in the market, but Applejack wouldn't have reason to visit Rarity's boutique. They respected each other's business acumen but didn't interact much socially.
21. What do Twilight and Spike consider their relationship to be?
I don't think either of them really think about naming it - it just is. It's just a natural part of the way the their world works - the sun is hot, the sky is blue, Spike and Twilight belong together.
She calls him "number one assistant" because young Twilight Sparkle was a pony who liked labels. Since Spike's not family (because they aren't blood relations) and Spike's not a friend (because friends are a frivolous waste of time), she created a special category just for him.
As for Spike, he doesn't care - as long as he's with Twilight he's happy.
22. When did Pinkie move in with the Cakes? Is she a worker who rents a room, an apprentice, or is there some other relationship?
Pinie left home not long after gaining her cutie mark. She went to Ponyville first because it was the closest town.
Pinkie seems most like an apprentice to me. She clearly works for the Cakes, but she's much too close to them to be just a regular employee.
24. Why didn’t the CMC hang out more/know each other before Call of the Cutie?
There's more than one class of foals in Ponyville Elementary. Scootaloo and Sweetie Belle were originally in a different class than Apple Bloom.
25. Is Scootaloo an orphan? Will she ever be able to fly?
Scootaloo's not an orphan. If she ever does fly it will be with assistance - like a pedal copter or hang glider.
26. Will Apple Bloom’s cutie mark involve an apple in some way, even if it’s unrelated to farming or baking?
Yes an apple will be part of it somehow.
27. How is Sweetie Belle’s relationship with their parents different than Rarity’s was?
Sweetie's not as embarrassed by her parent's bourgeoisie ways as Rarity is. So she doesn't work to distance herself from them like Rarity does.
Side and Background Characters:
28. Is Mr. Cake the father of the Cake twins or not?
30. How did Prince Blueblood get his title?
His family bloodline dates back to the old Unicornia royal line. His being Celestia's "nephew" is an historical fiction.
31. Is Silver Spoon equally as bad as Diamond Tiara?
Silver Spoon is just as bad as Diamond Tiara morally. Although Silver seems to be more follower than leader so she's less harmful when alone.
32. Is Zecora’s magic common among zebras?
No, Zeccora is a shaman/wise woman. It's a special course of study, not one most zebra follow.
35. Does Equestria differentiate between speaking and non-speaking species, in terms of respect, rights and legal responsibilities? Is there some other line or scale used? (For example, how are cows classified? Diamond Dogs? A chimera?)
If a species can talk, it's legally people. Some species that can talk may nonetheless choose to live outside the bounds of society (most Dragons, for instance).
36. Do other speaking species form their own nations, or are all nations largely integrated, with some having larger populations of a species than others?
Most nations are predominantly one species, though not every species has its own nation.
38. Do all zebras rhyme?
No. Either it's a trait common to shaman or else its a personal geas of Zecora's.
39. Do zebra cutie marks work the same way as pony cutie marks?
No, in the case of zebras it really is a butt tattoo.
40. Do any species have types of magic, other than ponies and zebras?
No other species has widespread magic talent like ponies. Dragons are also inherently magical, but their magic while powerful isn't nearly as versatile. Other species have a few specialists who practice magic the same way zebras do.
Headcanon and You:
42. Do you consider official sources other than the show (comics, card game text, movies, novels, etc.) to be canon?
Generally I only consider the show itself to be hard canon. Other official sources I consider to be "soft canon" - that is subject to being overruled at anytime by DHX.
43. Do you have one headcanon that you always use, or do you switch depending on projects?
One singular headcanon. I try to figure out how Equestria works and stick to that.
44. Can you easily accept the headcanons of different writers? Do you prefer when stories match your headcanon?
I have no problems with other people's headcanons as long as they don't contradict the show.
46. Is there a popular piece of headcanon that other writers use that you dislike?
Homeless orphan Scootlaoo is the biggest one. Ponyville's a small town and there is no way a kid could live on the streets unnoticed for years . Once noticed there's no way the situation would be ignored - not in Equestria. And certainly not by decent ponies like Applejack and Rarity.
48. Does your headcanon influence things like your favorite pony, ship, or episode?
49. Would it bother you if the show proved part of your headcanon wrong? Any specific piece that would really bother you to lose?
Not really, I'm more concerned by story quality. If the story's good then headcanon can go poof with no remorse.