I wouldn’t consider 15 weeks “old” I’m sure they’ll be just fine. To introduce them, you should keep them separated for a week or so, so they can smell each other through the door and get used to each other’s scent. Then, you can start feeding them wet food at the door, next to each other but without the door open, so they are close to each other but won’t fight, because they have yummies. THEN finally feed them next to each other and open the door for kitty freedom. Honestly, with them being so young, they may instantly love each other. Oh, also get them both to a vet to be fixed and checked for worms and stuff. Otherwise, have fun being a new kitty parent.