I would assume, however, that most 21 year-old young men had fully matured senses of their sexuality, and take into account whether the seventeen year-old in question could cope with that. Boys also tend to mature more slowly than girls so if you have a mature 17 year old they could really be at about the same level, sexually as well as mentally. Though as many have said, it totally depends on the personalities involved. When I was , I had a very defined sense of my sexuality even though I was a virgin until almost 18 , and I resented that people would think that just because I was a young girl I would have no sexual desires of my own, that any sexual situation I might get into would be the result of pressure from the man. I would probably not have a problem with my 17 year old daughter dating a 21 year old unless they were obviously on totally different levels of development.