Being bi or gay doesn't mean you are necessarily attracted to ALL members of one sex or another. I'm pretty sure your attraction to him comes down to something more than being "nice and cool" - there's most likely something else about him that drew your attention to him.
How can you put an age on it? if a girl is in her early 20's and has NEVER done anything intimate with anyone...aren't her first few forays into that still going to be considered experimenting? The whole concept of experimenting is not about age, its about figuring out what you like.