No one is too old or too young to be a Christian. It has nothing to do with a person’s gender, marital status, social status, height, weight, or age. Being a Christian is an issue of the heart where a person has trusted in Christ and wants to be like him.
Again, you are never too old to trust in Jesus as your Savior. As long as you are alive, you can call on Him to forgive you of your sins. He is as close as the call of your heart.
(Granted there may be some who are too young to understand the Gospel message, but here we will address those who simply use that as an excuse.) Youth is a blessing from God. Don’t use it as an excuse to stay away from Him. If you can understand what sin is, and your need for deliverance from it, then you are not too young to receive Jesus as your Savior. He saves everyone, young and old.