You truly want to marry each other...but others say you're too young. Who would u listen to?

There is a difference between taking others opinions into consideration, and letting their opinions run your life. Yes, you are young. But go ahead and get engaged. Just make it a long engagement. If you take 2 years to plan your wedding, that puts you at 23, and gives you 7 years together as a couple. That should be good enough to satisfy anyone's doubts.