River Song reached 

> 
> her final incarnation with Alex Kingston in "Silence in the Library" where she dies but at the start of the same incarnation in "Let's kill hitler" she doesn't know who The Doctor is.
> 
> 
> 

However, she often refers to how young the doctor is but assuming that Alex Kingston is in fact human, she will only be able to convincingly play the part of River Song for another few years without aging.

Unless the doctor suddenly gains an extraordinary number of years off camera then why does she often refer to how young he is?