Board Thread:Questions and Answers/@comment-29752445-20170928174955/@comment-1327106-20170928185608

Reiner might not know Ymir's age, but he probably has guessed that she's a transformed Eldian from Marley, and if that's the case she knows something about the Nine Titans.

In fact, if he doesn't know her age it's more likely he'll assume she knows about the current Beast Titan, which he does.

Zeke's ability to transform and command Titans isn't hidden. It's openly used as a strategy in Fort Slava and Colt brings it up in conversation. The reason he can do it when no other Beast Titan is hidden though.