User blog comment:God of nothing/Titan shifters-Invincible?/@comment-17563801-20131001222038/@comment-179280-20131002173303

It's possible that Ymir stole the "power" before, meaning she stole the serum or whatever it is and became a Titan but it's actually possible too that she became a Shifter after eating Berik (and she knows what she did, she just doesn't remember). If you remember, the Titan that worshiped Ilse looks like any normal Titan, and if Ymir actually stole Reiner's people's Shifter thing, he would have looked unusual in some way (because he was from her tribe and we are assuming she used the Titan power on herself and her people, hence making them all Titan Shifters).

And by aging, we know that they age normally when they are outside their Titan body, as we see Reiner, Bert and Berik as kids when they encounter Ymir for the first time (assumed to be year 845), and also the first two again as kids in Episode 2 of the anime.