I'd put mane7 to 13-17. In fictional world, responsibility doesn't necessarily mean maturity, but that's just my own belief. Here's my list (with birthdays):
Pinkie Pie-01.03.2000
Rainbow Dash-10.09.1999
Fluttershy-14.11.1998
Derpy-29.12.1997
Twi-17.03.1997
Applejack-08.06.1996
Rarity-25.02.1996
To everyone who's going to argue about Derpy's 15 years according to Dinky: Dinky isn't biological daughter of Derpy at all, because she's an unicorn. She's likely adopted.
CMC, I think, are no older than 9:
Scootaloo-30.11.2003
Apple Bloom-28.05.2004
Sweetie Belle-19.11.2004
Babs Seed-07.01.2005
Dinky-03.05.2007 (adopted 07.04.2008)
The older ponies:
Big Mac-1985
Mayor Mare-1964
Mr&Mrs Cake-late 1970's
Shining Armor-1982
Fancy Pants-1977
Oh, and Granny Smith is pretty damn old. 1920's
That's my belief, you don't have to agree with me.