Whenever I ride for a like 30 minutes around my time, my butt starts hurting for some reason, and I cant keep riding cause of it. Yet, I still WANT to. Any reason why?
I am a 12 year old Unicycler
What kind of uni, seat, style, terrain, etc… How long have you been riding? If you don’t describe you situation I can assume you have no seat and are sitting on the post. Anyone’s ass would hurt after that.
Get a good seat, wear some padded bike shorts, play with heights and angles of the saddle.
If you just starting out, it happens. Just keep at it.
Not sure of kind of uni, seat, or style
I’m riding on down hill rocky terrain, and I have been riding for 3 years but I stopped for about a year.
Before it didn’t hurt, and I could ride for hours, and hours. 
Find a picture of your seat online and post a link to it.
Why does your butt hurt? For example, if I were riding on a post without any seat, it wouldn’t be my butt that was hurting. 
Be specific. Is it the bottom edge of the seat chaffing away at the side of your leg? Scrotum issues? Or just plain crotch. Most of my wear & tear occurs right at the join of my leg to my crotch.
What are you wearing? Bike shorts are best. Jeans with thick seams are worst.
Why does your profile say you’re 22, and what are you doing in my bathroom? If you’re going to hang out in there, you should do some cleaning. Supplies are under the sink. 
+1