What do you do if you suspect a potential client is a minor?

[curiodraco.livejournal.com]

I apologize if this has been covered before, but I couldn't find anything searching the tags.

First off, as far as ways to rule out if someone is actually a minor or not..
I'm pretty sure you can't get a paypal unless you can verify to them that you're 18+? Correct me if I'm mistaken. 
In the US, can a minor get their own bank account or initiate a money order? 
I won't deal in cash, not only because of just the risk of cash in the mail, but that would be the easiest way for a minor to buy adult art.

Even so, with ways to rule out suspicion, I'm sure a clever kid can find ways around it..

So, what do you do if you deal in adult art, and you suspect a potential client may be a minor?
And how can I protect myself should I unwittingly deal adult art to a minor?

Thanks for any feedback :3

EDIT
Well that was easy. I should have thought to just ask the person to confirm they're 18+ but it didn't really occur to me because I'd hate to cause any upset for the inquiry. But I'm sure I can find a tactful approach to it without coming off suspicious, should I ever have need to. 

I guess it seems that there really isn't a good way to rule out if someone is actually a minor or not, and by asking I'll at least have a record of their confirmation, and then it's not on my back if something happens. I hope :s