[Kirarakim]

I guess you missed the part where

Spoiler:

Ed can no longer perform alchemy something that has defined him throughout the series

And so what that they all get what they wanted. That doesn't mean now life is easy for them. And actually they didn't all get what they wanted

Spoiler:

Izumi still cannot bear children, Roy is not the leader of the country, the Chimeras did not get their bodies back yet.

As for Roy getting his eyesight back he lost it so unfairly in the first place. He did not perform human transmutation by his free will.

So chances of Mei being around 10 are not that small.

I think they are very small because we see how young children act in this series. We had Nina and Elicia for that. Mei is an expert alchemist and she traveled on her own to another country. She was obviously a bit older than her appearance indicated. I would say she is no younger than 12. Either way though we were never given her age so I will think what I want and you can think what you want.