I believe the seller has a major responsibility in this situation, and I think educating him and getting him to co-operate is the best chance of reducing the occurrence of minors keeping venomous snakes, not legislation.
See this disclaimer in an advert found on a popular US site.
"Disclaimer: not to be a jerk but if we have any reason to believe that you would not be qualified to safely care for a rattlesnake we will refuse the sale. As with any animal we sell, we require that you are at least 18 years of age to purchase. Thanks for your understanding
"
He covers both important points there, first (and most important if you ask me) the fact that he may deny the sale if he feels you are "not qualified", IE age doesn't matter, then also mentions that no sale will be made to under 18s.
In my opinion this is the ultimate approach. The idea being that the seller sacrifice the sale if he disqualifies the buyer. If a seller builds a reputation for responsible selling it will serve his interest in the long run.
FYI - Other disclaimers (there are laws in US regarding the keeping of venomous snakes)
*If you need a permit in your place of residence I must have a copy before I ship! No Sales to Minors, 18yrs + only! Shipping through Delta ONLY*
*Venomous Reptiles sold ONLY to Legally Permitted Adults & only where venomous reptiles are permitted.. *
*No sales to minors. Proof of age may be required at my discretion.*