I believe I've seen a few 16 year olds. they just need written approval of a legal guardian.
My personal opinion? Unless they've done a lot of karting, make them wait a year or two. I sure as shit didn't have any clue about driving in crowds or using my mirrors properly when I was 16. It was overwhelming just getting used to the car, learning how to judge distance, etc. I'm not saying they won't be ok, just that having that year of driving experience is going to help a lot.
Now, if you do decide that it's worth letting them try, I recommend some sort of marking on the back of the car. When my sister drove with us the first time we put a "First Time Driver" sticker on the back window to give a little warning to approaching cars. We then covered it when experienced drivers were in the car. We also coached her on how to drive predictably and safely. She's coming back for NH, and my brother will be driving for the first time as well. We'll do the same thing. Rookie sign in the back window (but this time with LEDs for easier on/off) just to help other cars know why our car is suddenly driving at a different pace.
14 Time Loser FutilityMotorsport 2008 Saab 9-5Aero Wagon
Retired - 1989 Dodge Daytona Shelby
2011-2015 "Lifetime Award for Lack of Achievement" IOE, 3X I got screwed, Organizer's Choice