I think something some breeders need to consider is how enforceable is your contract? I've known cases where breeders and owners had disagreements and the breeder wanted to reclaim the dog, but had trouble doing so based on the contract and finding actual LEGAL BASIS (when actual legal ownership of the PROPERTY, which is what the animal is, lies with the owner, who is then free to do with it what they see fit) to reclaim the dog - and it cost the breeder thousands of dollars to even attempt it.
In terms of having people under 21 approach certain breeders - a lot would depend on the breed - I'm not sure I would sell a doberman to someone under 21 in many situations (despite me getting my dobe at that age) - probably not to a college student, etc - because as I've said in other threads, while owning a dog requires stability, it's a lot harder to rent and find housing, insurance, etc with a big, banned breed like the doberman (or a rottie, a pit, etc) than it is for a beagle, westie, etc - public perception and such DOES play a role in this. Stability in the life does play a role here.
In terms of giving my SSN/DL - probably not.