The only way I can see is very cumbersome.
Fax a copy of legal ID. Then Skype (with live camera active) the vendor. The vendor compares the photo id that was faxed with the person on skype. Of course that is time consuming, cumbersome and a problem for people with no live camera (Don't know about you guys but my ancient equipment wont do it).
I remember in an online virtual world I was involved with they went to an age verification system where you filled out a form and they used a big data base clearing house firm to verify age. It was such a fuster cluck that my cat could get verified (she was 19 btw).
Then, what they are going to be making is a very serious underground environment for vaping sales, in the end, the government will be hurting themselves and us all. The costs will skyrocket and would prolly be more expensive than a regular analog cigarette by the time it's all done and over with. (I believe, their true core intention)
There are SO many putting down the analog cigarettes and i'm pretty certain the government is feeling it in the tax department from tobacco sales dropping off. They are going to have to get those taxes from somewhere to develop their 'pork' plans and wasteful spending that our fat government is so used to having... (personal opinion, sorry)