This makes me feel uncomfortable, and my feelings would be the same if it was the lad who was the elder.
At the age of 16 the boy is still a child in the eye of the law. There are things that he cannot do legally, such as purchase alcohol and watch designated 18 films.
I am perfectly aware that many 16 and 17 year olds do do those things- as did I at that age
.
However, when I was that age, my parents were legally responsible for me. I didn't openly do those things in front of them, but they were aware, more or less, of what I was up to, and obviously made the consideration that I had the maturity to handle it.
The trouble with this age gap at that age is that if the boy were to want to do such things when he is out with the girl, it is her who has to make the decision whether she will condone it because she thinks him mature enough, or whether she will try to stop him if she feels he is not. This sort of power imbalance is not a healthy basis for any relationship.
There is also the issue that although she may be completely disinterested in doing anything that would be illegal for him, and he may be exceptionally mature, it is very unlikely that this would apply to all their friends. IMO it is an important part of any relationship that your partner is comfortable spending time with your friends, so a relationship that excludes them from their friendship groups would again not be the most healthy.