I can tell you one thing, the fact that you feel bad about it is a good sign that you are in fact NOT a monster. The thing is, there is no way to know what age someone is on here, so all we can do is assume that people are adults until proven otherwise. You did the best you could to help someone in whatever way you were able because that's the kind of person you are.
Based on GoodPersonEffed's response, it probably doesn't happen all the time, but even if it did, you can't control what other people do and it isn't your fault that people lie about their age sometimes to get in here. It happens on occasion in most adult-oriented communities and all you can do is report them when you know about it.
Hopefully you can find a way to stop beating yourself up over it. Life is hard enough without us making it worse for ourselves. I wish you the best.