^^ I agree with you that generally skill level/rating is more important than age and even physique, but I tend to think that this is only "true" when talking about people who are over 15 (maybe 16). Yes, he could be 5.7 and 150 lbs as you say, but I wouldn't recommend something like the K95 even then.
It's not the weight I'm concerned with really ... it's the other specs.
I wouldn't hesitate to recommend a MG Prestige MP to a 13 year old boy (even if it's 12 ozish) if he can really play and I know a 15-16 years old junior who can whip my arse with his Flexpoint prestige MP, but you will agree that the specs are still
a bit different.
A close to 70 stiffness coupled with a 340 SW are the specs I wouldn't recommend to any 13 years old regardless of his size. There you have it ...