Five year age gap?

One of my best friends is five years younger than me. He is sweet and wise and I've never met anyone who tries as hard as he does to make me smile even if he doesn't feel like smiling. I can't date him right now though because we're both still (for my personal preference) a little too close to the younger side of the age spectrum. But I could definitely see it at some point in the future, if it were to work out that way (you never know, these things always find a way). Would an age gap like that be hard in a relationship? Do you have an age gap in your relationship, and is it noticeable?

For those asking our ages, he's 17 and I'm 22. Legal, but I'd still feel icky.