Question: Throughout the movie Samara is about ten, but she comes out Noah's television as someone who is about twenty. Why the difference?
Answer: She is still in her 10's - the reason why she looks older is because her corpse is rotten.
Malaver
I've noticed that too. It is mentioned in IMDB that this scene was played by an uncredited stuntwoman and not the actress who played Samara. That actress probably was older.