First, to respond to Aurien, no, using a picture will not work, at least, as long as they work the same as pretty much every other modern facial recognition system.
But I am more concerned if, say, a young adult who was 20 was misidentified as a 15 year old and refused smokes. This would really, really piss people off (smokers can be very attached to the little fags, it seems). 
The only way I can see this working at all without causing anger is if the machine required a piece of ID (ie, drivers licence) and a face scan which would be compared with a previous face scan taken when the ID was issue, to see if they match. This way, one couldn't simply borrow a piece of id to swipe in the machine, as their face wouldnt be same as on the ID. 
But this would require facial scans to be taken with every new ID issued, which would raise even more privacy issues, cost, etc, etc. 
And all that to do what a guy working the midnight shift at 7-11 does; compare the looks of the guy presenting the ID with the picture on the ID. 
I wonder which would be faster? (Ive had than undeniably grade-school droppout stare at my ID for >5 seconds on many occasions....)

Can it tell the difference between a picture and a live person? That's the first thing that pops into my head. Just hold a picture of your 70yr old grandpa in front of it. If it can't tell the difference then it'll easily be fooled.