I'm going out tonight with a guy... it wasn't called a "date" so I don't really know what the story is.

But I decided I definitely don't want a boyfriend or do anything serious with anyone. I've been thinking and realized I'm not very mature and I used to try to hang out with older people but it just puts too much pressure on me to act different and sound cool and intelligent.
And being honest with myself most of the motivation for wanting to hang out with older people is because they can buy alcohol and I wanted to go to their parties.
The other thing is older guys that want to hang out with me are always guys looking for something serious or they think that I can make their life more exciting and like I don't mind having friends that are older but I think I actually prefer to hang out with people my own age because I have more fun and more in common with people my own age.

How do I explain this/when would be an appropriate time to explain this to him?

I think you should try make the date it as casual an experience as possible. Like, try not to treat it as a "date" date, just a casual meeting of two people with no intentions to start a relationship together, but rather to just hang out and talk and have a laugh. If you're assertive and don't show any signs of knowing that it's more than that, then the guy will comply with your attitude and it'll be fine.

How old are you btw?

18.

I don't think you should bring it up at all, just keep the date very casual and it will be fine. Just because you go on a date with someone doesn't necessarily mean you are expected to be looking for a boyfriend or a serious relationship. If he brings up the issue then you can very casually tell him that you don't think you want a boyfriend at this point in your life, but if he doesn't bring it up then don't bring it up. It would be really awkward if you did. Just hang out and be yourself.