It’s not how old we are in age or how long we have been a Christian that matters. Maturity in faith isn’t dependent on either of those factors.
What matters is how much we’ve yielded to the Spirit and surrendered to His leading in our life.
If anyone looks down on you because you’re young in age or in the faith (or both), let them.
It doesn’t matter what other people, including Christians, think about us. All that matters is what the LORD thinks about us. All we need to do is humbly rely on the Holy Spirit to live a Christlike life and set a godly example for others in the process.