User blog comment:God of nothing/Titan shifters-Invincible?/@comment-17563801-20131001222038/@comment-7418318-20131002091900

I'm not so sure about that. She didn't remember eating Berik but she did remember "stealing that power".

For me this makes it sound as if she stole that power when she was still Human and when she used it the first time, she couldn't control it. Just like Eren's 2nd transformation, where he also ended up as one of those brainless Titans until Armin managed to get through to him with the right words (and a sword-cut).

Also she said that Reiner and Bertholds people have sentenced her to death. They would hardly do that for eating someone in Titan form without conrol of themselfs, but they'd probably give that sentence to those who steal their ultimate weapon and likely only means of survival outside the wall.

Maybe those outside the wall developed the same serum as Grisha Jäger and that's what she stole.

As for Berthold being 11 when Maria fell, we don't know how fast or slow Titan Shifter age, once they awakened their ability. I wouldn't be surprised if their regenerative abilities (which they also have in their Human form to some degree) would also slow down the aging process.