NO AGE!!!!
This takes forever to cover for me.
Originally the Mane 6 were intended to be 4+ Years Old.
They had come up with this age system
stare Based around one type of real pony. In this system you are a Young Adult at 4 years old, but not completely an adult yet.
The only other things I can gather is Rainbow Dash is likely the Youngest. Fluttershy is the oldest. Applejack is likely the second youngest. Pinkie Pie is 1-1.5 Pony Year younger then Fluttershy.
If you want me to try to come up with some conversion.
I would assume that 1 pony year = 4 human years
Gathering everything I can.. and they information I just shared.. I think it would go something like.
Rainbow Dash was about 5 during the Rainboom. Fluttershy was about 15 during the Rainboom. Pinkie Pie was about 10 during the Rainboom. The others were between Rainbow Dash and Fluttershy's age.
During the Rainboom I would put them all about 6-12 years old with exception of Rainbow Dash and Fluttershy.
Pretending Rainbow Dash is currently 16. I would age them all 11 years when MLP:FIM starts.
That would put Fluttershy at 26 and Pinkie Pie at 21.
That would put Applejack at about 17 or 18.
I am using the Rainboom to also help things here.
During the Rainboom Rainbow Dash and Twilight Sparkle instantly received their Cutie Marks. At this moment we see all of the ponies. Fluttershy already has adult eyes. If you look at the eyes on Ponies they actually have a system of lighting to indicate their age. Fluttershy was clearly a Teenager when the Rainboom occured by combonation of her appearence and the light count in her eyes. Although Applejack was " one of the oldest" of her class to get her Cutie Mark, she did not get it instantly when the Rainboom happened. She had to complete some things in another city and then return home. So while Applejack was probably 6ish by Human Years when the Rainboom happens, she was probably 9 or 10 ish when she got her Cutie mark.
All of this does not give you an exact age on Rarity and Twilight Sparkle. The things said about Rarity conflict. There were comments that she was one of the first in her class. There are comments that she was late. There are comment she was one of the last. Things are inconsistent with Rarity and since she Drama Queens she will exaggerate. All we can gather is she was not the oldest nor the youngest. If she was, we would get more clear information. Since it's inconsistent, she was somewhere in the middle. Of course I mean in relation to her class. @_@;
For more dramatic inconsistency, you can poke at Cheerrilee. Who has to be like 40-50 human years old
rofl
But, yeah,.. if I do a ton of math and other junk the Main/Mane 6 would be 16-26 human years old.