Localbitcoins.com for in person trades. It's strange that the sites want you to be 18 to buy some.
Actually the best way to get some is to do some work and get paid in them. Might not be too realistic for you depending on where you live though.
I would guess it's done for legal reasons on most.
Just seems like if safety is a concern, allowing kids to buy them online would be preferable over forcing kids to meetup with strangers. *Meet in a public place like a coffee shop!