Around the time Eren was 5 he could talk pretty well for his age. All of his siblings could talk well beyond there vocabulary range, and Carla had called them all geniuses and prodigies.

It was around this time that Mayoral Elections were being re-held for the state he lived in and Carla was running for Mayor again, she insisted that she wasn't done flushing out the police of bad apples and helping reform the high school systems to teaching more practical lessons. 

And it was around this time that there were many attempts at her life and attempts to kidnap her children, all only being thwarted by conveniently placed windows and nosy neighbors.

So Carla decided she wanted her children to learn self-defense so that she could worry, if only a little, less.

It just so happened that the dojo that Carla picked for her children to learn from, was host to two more titan shifters. Annie Leonhart the Female titan and Reiner Braun the Armored titan.

Eren couldn't lie and say that seeing them didn't send bristles down his back like a cat about to attack, but he was a lot calmer after spending 5 years with Ymir and Bertolt.

The first few encounters were pretty stiff, but after spending more time with everyone Eren became accustomed to being in a group with all of the Titan Shifters. About half a year had past until he noticed that there behavior around him was shaky and stiff as well, and after confronting them about it, they all found out that each other remembered.

All of the Titan Shifters remembered there previous lives.