I must say if I had a daughter of this age wanting to be with a man of my age I would not let this happen. I also know how I feel for her now and what I believe. I would not know how he feels or if I could trust him either. So I do see where they are coming from.
When I was a 16 year old I would have no problem being with a 21 year old woman. I had plenty of different things on my mind back then though. I learned to control my self from back in my teenage days though hehehe. I do most of the same things now that I did when I was 17 or 18 but I am still a little young (besides the going to work instead of school and the paying of taxes). I guess this will change in time also. (Not paying of taxes but the other activities)