You can never prevent kids from buying what they want, or get their hands on what they want. Having a credit card purchase should be enough to prove you are 18+. If they get someone to buy it for them, the purchaser takes responsibility for the kids having it. It's the same goes for buying liquor, or R rated materials.
Online store can only really do so much to insure the client is 18+, a credit card IMO should be enough.