It’s a great thing that you and your boyfriend started as friends before committing to one another!
Don’t worry about your age. Age is something that shouldn’t be used as an excuse or judgment tool. We are all young at heart. What matters is how you see yourself. You’ve been in a committed relationship with someone that respects and cares for you. Some older people, my self included, have yet to experience that. So be proud.
Comparing your self to someone else is hurtful and not very useful. Don’t worry about how different or alike the other girl is to you. Think about it with an open heart. The great thing about poly-lifestyle is that you don’t have to be anyone you are not. If you are red and she is green then he gets a more colorful life. Same thing with the guy you are interested in. I am sure there are things that make each of them different and that makes them both fun.
Take it slow. You don’t have to jump, you can take small steps. If Poly is for you, you can try it another time when you feel more ready. Only you can know when is the right time to take your relationship to the next level.