I feel like 15 gives them justice, for the most part. 16 feels too old and I'm frankly appalled at suggestions of 10. x__x
N does feel a little older, though. 16?
But you don't exactly "know" people when you're a baby, right? This implies to me that some years could have passed before they came to know each other. And hey, they could have met when they were 6 years old or something, which would make them 16 years with that statement... :o
Cheren: "Bianca, I've known you for 10 years and..."
^ He says something like that in the beginning of the game.