My favorite thing about taking classes as an adult ("adult") is that the grades are an auxiliary nuisance that I can ignore if I want to.
I'm about halfway through this PMBA program at Wash U—27 credit hours done (at the end of this semester) out of 54. PMBA just means that the class includes people who also have full time jobs and take MBA classes at night, i.e., everyone is an adult ("adult"). This first set of classes was entirely required classes—introductions to accounting, finance, operations, marketing, etc.—the basics.
My least favorite thing is that so many people in the class get hung up on the grades for each assignment, each quiz, each exam. It doesn't matter. The final class grades aren't even letter grades, but some kind of rough pass/high pass/low pass/fail distribution that is surely weighted towards a regular old pass. To fail, you'd probably have to be not just delinquent but destructive. And even if the grades mattered... so what?
For me, taking classes as an adult is a gift—I learn what I want to learn, study what I want to study, I do well, but I don't get hung up on the evaluation system. 90% on an assignment is the same as 9% to me. It's fun and interesting to figure things out. It's a game. And I think you can have more fun and learn more if you approach the class like that instead of trying to negotiate for points on things turned in. Do I need a degree or formal classes for that? No. But, like paying to enter a road race versus just going out for a run, aligning myself in a game with and against other runners produces different results.
The game was different in college as an undergraduate. Grades had some (short-term) consequences that could affect your (immediate) trajectory. But I think I would give as advice a modified version of what I believe for myself now: dig into the things that interest you. Receive an interesting bit of knowledge in a lecture or textbook or assignment? That came from somewhere. Track it down. Learn how the people who figured it out figured it out. Create a model or code or whatever to extend that knowledge into something real or practical (or real and impractical).
The grade is just an ornament. And not a very substantial one. The knowledge itself is where it's at.
Side note: an all-time good post from Seth Godin: The wasteful fraud of sorting for youth meritocracy: Stop Stealing Dreams (2014-09-02)