Actually, when I was younger, I would always stay up all night so that I could get over jetlag easier. (All night gaming session! What 13 year old wouldn't like that =P) Not saying that this works for everyone, but it worked for me great... The idea was that I would get on the plane and pass out immediately (matching the timezone) and then wake up later on in the flight.

If you are sleeping, they will try to lightly check you, but overall they tend to just skip you... I have slept through quite a few meal services and politely asked if I could get it later without much trouble.

Overall, there is not much harm that jetlag can do to a 13 year old unless he has some kind of competition awaiting him when he arrive in PEK. So, I wouldn't over think it either!