Communication is everything. As long as everyone is completely up front with how they are feeling. Honest about their motivations. And respectful about their partners. This can work out fine. As soon as people are keeping things hidden, that's where disaster lurks.
There's no age limits on threesomes, you know.
I don't think citing age is a sound reason for this. And I might be over analysing. But it seems to me perhaps to imply that you're under confident about the sex you've already had. If you don't feel like you've had enough experience, then a threesome is one way to remedy that. It does risk leaving your partner feeling like they "aren't enough" for you. That's what I mean about communication and honesty.
Also, I'm making the assumption that you'd want a second woman. How would you feel if your girlfriend was willing to do this on the condition that you also have a threesome with a second man? Are you willing to go along with her fantasy in order to fulfill your own? It might not be an issue, she might be interested in this herself. Just throwing it out there that if she's willing to do this just to make you happy. Again, that's a disaster scenario in waiting.
It's really important to practice safe sex to protect all three of you. And ideally, to meet beforehand and discuss the details. Some people have drunk, opportunity threesomes. Those are much riskier, though.