When I was 21, I considered 16 year old boys as children. Still do, even the most intelligent, mature, and sensitive among them (all of whom which post here 😛 ). I had already been initiated into the adult world, and that makes all the difference.
This is really a sociological question, and the answer depends on the norms of the society you live in.
Perhaps in a primitive society where 16 year old males are already hunters and perhaps a 21 year old female is available as a mate, it wouldn't make a difference. Both have been initiated into the adult world. However, in a modern society, people take much longer to mature emotionally and mentally. I suppose there are some exceptions, but the bottom line is, a responsible citizen will obey the laws and wait for someone to at least be of legal age (if not mental and emotional, hah).