Hasbro assumes that the first 3 seasons were 1 year (mathematically possible)
Estee assumes that 1 season = 1 year (more or less)
The key is the events of The Cutie Mark Chronicles
Spike hatched then, so the key to "How long ago was that?"
is "How old is Spike during the pilot?"
I'm going with "5 years old"

Going by this:

The Crusaders
There is an Apple family reunion during the pilot
During Apple Buck Season, AJ called it an Apple family reunion
During Apple Family Reunion, Granny tells Apple Bloom:
The reunions are every 100 moons (8.3 years)
The reunions are always at Sweet Apple Acres
This is your first reunion
AFAIK, there is no way to reconcile these statements

AB was at the gathering in the pilot.
If that was a reunion, she'd remember
AJ made MANY mistakes in ABS, including forgetting how big her own orchard was
I'm going with "Granny was right, AJ was wrong"

This makes AB no more than 8 during S3
Hasbro has said that that the Crusaders met during Call of the Cutie
Their cameo in the pilot was a continuity error
Their age in Call of the Cutie would be 5-6 & they were just starting first grade
I admit, this doesn't match well with Dash's claim

Applejack
During Apple Family Reunion, AJ shows some baby pictures from a previous reunion
She looks about 1 year old. If you assume that the pictures were from 2 reunions back (about 17 years ago),
then she was 18 in S3, 16 during the pilot, and 11 when she got her mark

Dash
Probably joined the Wonderbolts as soon as possible
This makes her 18 in S3, 16 during the pilot, 11 when she did the rainboom
If you assume that she got a job upon graduating primary school
(normal in the USA until post WWII)
This makes her 14 when she moved to Ponyville in A Series of Egotistical Events, 2 years before the pilot

Fluttershy
Dash tells the Crusaders that she was the first in her class to get her mark.
This probably means that she is younger than Fluttershy
(in A Series of Egotistical Events, it specifies that she does NOT know Fluttershy outside of flight camp, meaning that they were in different schools or different grades)
In Triptych, it specifies that Fluttershy moved to Ponyville about as soon as she legally could (14?)
& implies that this wasn't too long after she got her mark. So, she was 12-14 when she got her mark
and 17-19 during the pilot. This would mean that she moved to Ponyville 3-5 years before the pilot

Pinkie
In Griffon the Brush Off, Fluttershy says that she is 1 year older than Pinkie (give or take a bit, no doubt)
This makes Pinkie 16-18 during the pilot and 11-13 when she got her mark
In Triptych it specifies that she ran away from home within days of throwing her first party
(ICR if it specifies how long after she got her mark that took place)
Dr. Gentle took her to Ponyville shortly after that. So, she moved to Ponyville about 5 years before the pilot

Rarity
She doesn't have her mark in War Journal (age 8) & doesn't seem to be interested yet in theater or design
In WJ it specifies that she is in AJ's grade, so she is within a few months of AJ's age
(unless AJ was held back a grade?)
Tricks of the Trade Show takes place when she is 12(?) (whatever the minimum age to leave school is)
at the start of seventh grade (2 more years of school)
She got her mark at 11, came to Ponyville at 12 (In Autumn, about 3 & 1/2 years to the pilot)
and was 16 during the pilot. Pinkie and Fluttershy were already there

Twilight
She was applying for Celestia's school (9th grade?)
This would make her 14-15 when she got her mark. 19-20 when she moved to Ponyville during the pilot
This means that she graduated a year before the pilot. She must have been going to college & taking Summer classes
This is why her friends thought that she had time for a party

A Series of Egotistical Events doesn't mention Dash getting a Pinkie Party
Possibly, time/money constraints, maybe Dash living over town not in town threw her Pinkie Sense off

You put a lot of work into that. Well done.

6750422
Thank you. The only thing that I'm uncertain of is "8th grade education is standard"
(It was in the USA until post WWII)
If it's HS (12th grade) I can redo it but I'll need to know
"How old was Dash when she joined the Wonderbolts?"

6750444
8th grade is essential, you can drop out of high school, but it's not reccomended, so I'd say the full 12 years is essential.

6750444
It depends as to the actual setting. I see the setting as illustrated feudalism rather than anything modern, and unless Canon says otherwise, I would revise their ages downward by no less than two or three years.

Do remember the fact that the meaning of the "Sweet Fifteen" party is to present the fifteen year old girl as a woman in age to be married. Or the fact that, in the play Romeo and Juliet, Juliet is supposed to be fifteen, yet her father is being pressured because most high society women her age aren't only already married, but happen to be mothers.

6750577
It would depend as to how far does universal education actually extends in Equestria. Or how far it extends even at the highest levels.
Or the formats it actually takes. Does Applejack, for example, actually know much more than her elementary education and farm lore?

Twilight also appears to have been plucked out of Celestia's school to become her personal student. She does apparently take some classes with her peers, but the lion's share of her school experience seems to have been a personal seminar.

And what would have been her degree from the princess's school? Going by olden standards, she's probably a Philosopher in Magic.

6750871
100% Move, 50% Fire specifies that she lives in the tower (as seen at the start of part 1 of the pilot)
Also, in Triptych, it specifies that Coordinator does his best to turn her classmates against her
It is also specified that she works in the library (at least part time) but isn't very good at it

6750991
How about Canon?

6751022
Estee doesn't use the AU label, but there are some significant differences
(If you look, Triptych was started just after Magical Mystery Cure but before the first EQ Girls movie & before S4)

Two notes:

Dash may have never got a full welcome party because she moved in while Pinkie was still coming out of her shell and accepting her mark. We see in Lazy River that it took Pinkie a good while to come up to full steam because her family really messed her up and she had very little socialization (likely zero with non Eath Ponies) despite one of the most social Marks.

IIRC in the Contiuum Dash actually finished weather school as well, although she is young for the job and mostly got it on sheer talent and Ponyville being a backwater rather than work experience.

6751161 "Dash actually finished weather school as well, although she is young for the job and mostly got it on sheer talent..."

"Ok, class. For today's exercise, we have five clouds over in the exercise area--"

There was a blue blur of motion, a blast of wind, and Instructor Downdraft stumbled at the edge of the cloud before he managed to catch his fall with a well-timed flap. By the time he landed on all four hooves, Rainbow Dash skidded to a halt next to him.

"Ok, I busted 'em. Is that all we had to do for today's class, because there's a Wonderbolt flyover in a little bit, and I wanted to get a good seat."

6751567

"-- and thank you Rainbow Dash, for volunteering to show us cloud-weaving, from scratch!"

ESTEE

If this doesn't suit you, I can redo it but I'll need some authorial decisions
1) How old was Dash when she tried to join the Wonderbolts?
2) What's the minimum age to drop out of school?
3) What's the legal age to sign a contract?
4) If the Crusaders are older, then where was AB during the last Apple Family Reunion? Off on a trip with her parents?

"Auk-ward…" tells us the following:

Applejack was tied for the oldest in the group: the same age as Rarity. It didn't mean much: there was a rather narrow spread of years between the Bearers.

But Fluttershy... she often felt as if she was treated like the baby in the group, even when she was a year older than Pinkie.

6750387

Dash
Probably joined the Wonderbolts as soon as possible
This makes her 18 in S3, 16 during the pilot, 11 when she did the rainboom
If you assume that she got a job upon graduating primary school
(normal in the USA until post WWII)
This makes her 14 when she moved to Ponyville in A Series of Egotistical Events, 2 years before the pilot

Egotistical Events states that Dash got her Weather Bureau job right out of "secondary school".

Drunk Little Ponies establishes that Equestria has a drinking age. Triptych (Craquelure) tells us that the Main 6 went bar-crawling after the Gala. So they all must be over the drinking age at pilot+1 year.

6909607
Well, I have since rethought this question. Hasbro goes with "the first 3 seasons were 1 year" but Estee goes with "1 Season = 1 year" & also assumes that everything after Magical Mystery Cure is non canon

IMO, Estee's Mane 6 are several years older than Hasbro's Mane 6
(Late teens+ vs early to mid teens). I'll post you the reasons why if you want, but it is rather long & probably not of general interest