That could be okay but you will be leaving high school sooner than he? You have to move into a life that he cannot imagine/experience yet and you probably should be working to relate to that rather than to what is enjoyable when you are/were 2-3 years younger. Do you drive a car? He cannot yet. There may be more of a "little brother" sort of thinking on your part or a "big brother" one on his and if he's looking up to you, as he probably is, what you do is affecting his life in ways neither of you may be aware of yet. Pretend you decide you don't care about school and drop out; he could take that attitude later since his older friend did or maybe he has a problem you know the answer to, he might become more dependent on older people instead of starting to learn to wrestle with his own problems. It's a big responsibility for you, being older, and it can be a danger to him, looking up to you instead of to himself and/or those in his permanent life/positions of authority, etc.

It is not built-in harmful but just potentially in very subtle ways. We get in the habit of learning and doing things a certain way as we're teenagers and while the age difference might not make a difference in your 30's or 40's, then you'll be more set in your ways. Now a younger good friend could be limiting for you as you don't "see" anything new, learn and grow, and it could postpone his self-awareness and "stunt" his working on his own growth.

I guess it depends on what you do together. If you both play an instrument in the marching band and want to get together to practice or something, that's one thing. But in any event I would make sure I had other friends/acquaintances and maybe get together with younger friends in a larger group so there are multiple points of views and others who can subtly affect the mix. Start or join a study or other group open to friends in your and other grades. Do things together with your father(s) or as part of a team or something. Play video games together or as a team but not exclusively (don't spend all day Saturday in a basement somewhere playing video games, for example).

But no. There's nothing wrong with being friends with someone younger/older, just think about what you want to be friends "for"? What are you getting from it? If you are escaping "scary" same-age guys who you feel you have nothing in common with, etc. you may want to rethink the "practice" needed to learn to get along with others our "age"/development level. I'm not saying you have to try out for the football team, but perhaps go to football games and talk to other students, etc.?