If that's really the case, you can follow a course on doing taxes. I'm serious. A 4-year degree is a big step, but a 'basic' course - doing taxes, or programming 101, getting your First Aid certificate, philosophy, foreign language - in something you find interesting, can get you back into the swing of things. It's also 'lower threshold' and not 40 hours a week.
Or a single course of a full degree?

I am in 'adult education high school' although I think my oldest classmate is 23 or something. (I turn 21 next month) However, I do day classes. Most older people go to evening classes. There's some mothers/fathers or people with a day job that need more education in order to promote. Military people. Once or twice a father and son doing one or two subjects together.

As for befriending a 40-year-old in class: I'm not good at approaching strangers. And making friends is not my primary motivation at school. But that's with all my classmates.
That said, if you were to sit down next to me and we get along: sure. If you can see me as an equal, and not as a 'I have a child your age'.

You might also be surprised at the fun you can have with teachers. They might not be your friend, but in adult education, the boundaries are less strict. You can tease them and they tease right back. In the beginning of the school year, when I didn't know anyone yet, that was enough to make me feel comfortable, welcomed, not-alone. In some classes I often sit alone; still I don't feel left out, because the teachers are just that little bit looser. And because there is no bullying (it's not allowed either, after one warning you're kicked out. If it's severe you won't even get that warning) I don't mind sitting or walking alone. (If you're getting bullied, sitting alone is screaming you have no friends, which means they'll target you even more. In that case sitting next to an indifferent classmate is actually preferable.

I tend to gravitate towards adults anyway.. have all my life. Because no fellow 8-year-old had ever heard of liberal and conservative, and I wanted to discuss the newspaper. *shrug* So as long as I am not looked down on because of my age, I won't mind your age.