Is it okay for a girl to date a younger guy?

Okay my cousin Daryl is 17 and she will be 18 April 1st and she is planning on dating a guy that just turned 15 in September. I told her that it won't work that he is too young and she compared her situation to me dating a 21 year old when I was 17. I don't think it is all that bad when the guy is older, not the girl..