Dating isn't easy at any age. First impressions are not always what they seem. When you meet a guy and you are interested and so is he, you give off signals. It's not always easy to tell if he has a girlfriend, especially if he isn't giving you that information. But there are some sure fire signs to watch for that may be good indicators he is taken.
You have met for coffee already with this guy and you really want to see him again so you ask for his phone number. If he is reluctant to give it to you and says he would rather call you, this could be a good indicator there's someone else in his life.
If he only wants to email for awhile and won't give you his number or take yours, this is another good indicator there is someone else in his life.
If his "free" time to see you is only during the week.
If his contact with you is sporadic, for example, once every two weeks, "Girl, it's time to move on. He's got a girlfriend."