Age Of The Main 6 During Pilot · 2:35am Mar 24th, 2022
I saw an episode of Cutie Mark Laboratories that covered this & was inspired to go into my thoughts on the matter.
The episode Point Of No Return established that all months are 28 days. 13 x 28 = 364. If you assume that ponies have an intercalary day (perhaps the Summer Sun Celebration) then their year is 365 days.
IMO, either is close enough to an Earth year to use to measure lifespans.
I think the first 3 show seasons cover 1 year because
Holidays
There are a great many shown and all are shown only once. There are no repeats.
Birthdays
Twilight, Pinkie, and Spike all have their birthday shown. No repeats.
Celestia
ICR the exact words but in Princess Twilight Celestia says something to the effect that the upcoming Summer Sun Celebration is the first since Luna returned.
Rainbow Dash & Tank
Dash got Tank in S2. In Tanks For The Memories (S5) She still doesn't know Tank hibernates. 
Meghan McCarthy (show writer)
When asked, specifically said that Equestria Girls was canon to the show. This has some problems (like "Why didn't Twilight know who Sunset Shimmer was?") but it shows the Main 6 as HS students (and thus 18 or younger).
During the S9 episode Point Of No Return it was established that all months are 28 days. 28 x 13 = 364 days. There is no evidence that they do but it is possible that they have an intercalary day (the Summer Sun Celebration, perhaps). This would make the year 365 days. I see no reason they would have leap years. This means that the EQ year is a little shorter than an Earth year, but IMO it's close enough that they can be used interchangeably for things like a lifetime's length.
Most bronies assume that ponies live a more or less human lifespan & not a horse's lifespan (25-30 years).
I'm going with the assumption that the first 3 show seasons are 1 year (give or take). I base this on
1) Holidays
There is only 1 of each holiday. Hearth Warming, Hearts and Hooves Day, Running Of The Leaves, the Gala, the Summer Sun Celebration, Nightmare Night, Winter Wrap Up, all happened only once.
2) Birthdays.
Those whose birthdays are shown only have 1. Spike, Pinkie, Twilight all have only 1.
3) Dash & Tank
Dash gets Tank in S2 but in the S5 episode Tanks For The Memories Dash is unaware that Tank hibernates.
4) Princess Twilight
Celestia specifically says it's the first Summer Sun Celebration since Luna returned.
Based on the number of days shown in the episodes, this is mathematically possible, especially if you assume
1) The CMC episodes take place during the Main 6 episodes.
2) Magical Mystery Cure happens in just one day.
Yes, the order of the episodes is scrambled. S1 E11 Winter Wrap Up is shown before S1 E13 Running Of The Leaves is the most extreme example but there are others.
The Age Evidence
1) In the S1 episode Griffin The Brushoff, Fluttershy tells Pinkie "I'm a year older than you"
2) In the episode Apple Family Reunion Granny shows a baby picture of AJ taken at a previous reunion. AJ looks maybe 1-2 years old. Granny says the reunion takes place every 100 moons. If you assume it was 1 reunion ago, that's under 8 years. This works if you assume they only have an Earth horse's lifespan but otherwise it has to be either 2 reunions (15 years 5 months) making AJ about 16-17 at that time. If you assume it's 3 reunions ago, that's 23 years 1 month & makes AJ about 24. AJ does seem more mature than the others especially Pinkie & Dash but IMO she's about the same age as the others so I'm going with 2 reunions.
3) Megan Williams (an important show writer) specifically said EQ Girls is canon to the show. The Main 6 are shown as HS students (& thus 18 or less).
This does raise the question of "Why doesn't Twilight know about Sunset? Was she that oblivious?" +The CMC are in the Battle Of The Bands (& thus in at least Junior HS (& thus at least 12) but 1 problem at a time.
In the S4 episode Pinkie Pride Dash says she came to town on her birthday. The cake shows 21 candles. I'm going with "artistic license". 18-19 for her age & the rest for anniversaries.
In the USA, you can join the military (Wonderbolts) at 18, 17 with parental permission. +Dash is in the YOUNG Flyers Competition. So, 16-17 during the pilot.
In Cutie Mark Chronicles Dash specifically says she was the first in her class to get her Cutie Mark, so younger than Fluttershy. There were some ponies with cutie marks, so there were older foals there. Gilda knew Dash from flight camp but apparently not Fluttershy. So, I'm going with "Fluttershy is older than Dash & Dash met Gilda the next year (or later)."
In the pilot, Rarity has her own shop & is in charge of decorating the town. In most of the USA you can be an emancipated minor at age 16 (14 in California). So, maybe 17-18 during the pilot.
If you assume Dash & Pinkie were also emancipated minors, they both would have come to town at age 16. Since Pinkie doesn't know when Dash came to town, I think Dash came to town before Pinkie & is likely older.
Being an emancipated minor seems more common in EQ.
Twilight has the fewest hints for her age.
So, Pinkie and Dash are 16, Fluttershy is 17, AJ is 16-17, Rarity is 17+, Twilight is ?. But, presumably at least 16.