Depending on the airline, joining too young can cause problems. I've found that I sometimes work with people who talk down to me because I'm only 23. A lot of people where I work (BA) started so long ago when people were very different that they don't understand how things work now.
I've been called over-confident and cocky several times now and I know a lot of new, young recruits who get this as well. In the past it was normal for the new/young crew to "be seen and not heard" so some older crew have a real problem with younger people, especially if they can show initiative and get things done. A lot of crew think that the job is the most important thing in the world and think that your experience with the company is the only thing that counts. Before I joined I was an engineer, I spent 3 years at university and before that I was a chef but to some people that doesn't count and they show they don't like it. Maybe this is only a problem in BA, I really don't know about other airlines.
I warn you that an airline is not a great place to meet lots of life-long friends, just thousands of acquaintances. In the year I've been flying I've met one person who I see outside of work, but everytime I report for duty I stop for a coffee with someone I've met around the building.
The good news is that things are changing (they are at BA anyway), the companies are recruiting a lot of younger people and some of the older crew are being forced to accept that times have changed and it's not only the "old hands" who can do the job well.
Best of luck with everything, BA are currently recruiting so give them a shot, but I also hear that Virgin are a lot of fun to work for (although I may be wrong, anyone from VS here to comment?).
Feel free to PM if you'd like to talk about it further.