香港人到大陸網吧一問

I may interpret the reasons / scenario as follows:

1) According to your info, your friend ought to be permitted to enter. The mainland policemen had no action probably because your friend comes from Hong Kong and not mainland. Their enforcement is mainly on mainland youngsters, so when your friend showed HKID and they understood he comes from HKSAR, they let him go. It does not mean in another situation, with another group of policemen, he can still remain there without problem. This is only an exceptional case in my opinion.

2) About the exact year as adult, the official year should be 18. It can be easily illustrated by a few examples:

--> Only a person with 18 years old can apply for an Adult HKID.
--> Only a person with 18 years old can register for elections.
--> Only a person with 18 years old can have sexual intercourse without prosecuted by police upon detection.

Those 16 and 17 may refer to the age for work. For instance, when you are over 15, you can work as a full-time worker without trouble. But the law has banned the kids without 15 years old to work FT, in which they can only work for PT.

Hope I can help you.