I am not eighteen yet so I can't play online poker for real money. However, I want to study and practice in the meantime so I can use poker to make some cash to spend in college. The problem is free online poker just attracts people who go all in constantly or people who limp in every single time. Are there any free sites with people who play at a decent level? Or maybe an AI I can play against with adjustable difficulty? Any other ways for me to apply the things that I am learning from studying?

Free sites will just teach you habits that you'll have to unlearn. Try to get some pals togther and play for something but where no one can lose so much that it hurts you financially (e.g who pays how much of the pizza bill)

If you still want a free poker site there's stuff like
gpokr.com
The small tables are shove fests, but once you work your way to the higher tables there are actually decent players around.
The small tables don't teach you to play...but they do teach you patience. (Observing the others it mostly teaches you how not to play)

One (free) app that you might find fun and educational is Poker Fighter. https://www.poker-fighter.com/
I'm not aware of any good AI tools that allow you to adjust the strength of your opponents. AFAIK, most of the poker bot training apps (e.g. Snowie) are either quite expensive, or are strictly for heads up, and don't have very good interfaces, but I haven't looked in the Android or Apple stores recently.

i practiced and honed my game at wsop.com
went about 2 weeks without a losing day.
terrible players
but beleive me its rigged. not 100% ramdom dealing.
when you get used to playing at a site like this, then switch to live poker you gotta remember:
the players that play live poker are 300% better then play money.
you will not see as many good hands live and not as many bad beats but you still going to see them live poker.
but its a good way to try different playing styles. see what works. then see what style you most comfortable with.

Poker Genius is pretty good. Its not all that expensive ( I think about 40 or 50 bucks to buy ).
It will allow you to play thousands of hands per month and you can toy with different styles. You can hit the little bolt looking thing under your name to skip hands you folded pre flop and move to the net hand.
I like to keep the advice button off as AS you get better your choice will often be better then the advice given. BUT The best part is it plays reasonably close to real live game conditions.
If you are serious about poker it is worth the buy .
In and of itself it is NOT the answer or ""the bomb" but its great to practice approaches you have read in good poker books.