Is it bad for a 17 year old to date a 21 year old?

I cannot say that it is bad, it all depends on how the two feel about each other. Although there is a thought that the 21 year old might be taking advantage of the situation, that might not be the case. What I can say is that: depending on where you live, it IS ILLEGAL for a 21 year old to be dating anyone under the age of 18. Doesn't matter if the 21 year old is male or female, it's illegal and considered abuse. So even if the 21 year old is true to the youngster, it will not matter.