My first LTR started when I was 21 yo and she was 17. It lasted for almost 8 years with no regret. 3 or 4 years is nothing nowadays. Girls tend to grow faster as everybody knows and girl at age of 17 is definitely not a kid.
As you get older a few years different doesn't make much difference. At your age, he's experience a lot more of life than you have and you have some catching up to do. A guy closer to your own age would be easier to relate to.