Not so much to do with age, but maturity.
Being a member of several forums I have seen many times the younger Generation in these forums. We had several teens in one of our groups, most of them acted like responsible teens and fit in nice. we had a few that acted like D/D wanting everything but never following through. and the sad part is if you sell them something without the parents involvement, you can loose your parts & the $$..in most states any contract with a minor is not valid, so if the parent says the minor used a credit card or pay pal account with out permission they will refund the $$ and the seller looses the parts & $$. So it's important to know who your dealing with!
I'm Glad to see the younger Generation getting involved in these hobbies, Without them this stuff dies with us.. but you need to use caution until the minor proves they are mature enough to deal in buying & selling.. let them ask all the ?? they want, that's how we learn. but if they want to buy & sell be sure you talk to a parent first...