not sure if it's a hurdle but being constantly referred to as an adult takes a while to get use to.
For a while I would cringe when I stopped being called 'girl' and it replaced by 'Lady, woman or worse Ma'am.' The joys of being in customer service meant I over came this quickly though (Mind you, being called Ma'am still makes me groan internally).
maturity comes with trial and errors in your twenties. high school didn't do sh*t, now is the time where you actually learn life skills and find out who you really are now that you gotta prepare to start living life without mommy and daddy holding your hand. the real world comes at you like a brick so while your 20's is the time to make mistakes and enjoy youth, you still gotta handle your business so that brick doesn't hurt so much when it comes.
Happy Birthday mate, your life starts officially from here on. The biggest change I felt after losing my teen-hood, is that you've gotta start taking up responsibilities, and you gotta start laying foundation for your career. Lot's of parties, drinking, women, hehe, strap on your seat belts brodha, it's gonna be a bumpy ride.
Not much. The biggest change is the first few years out of high school which you've already had. That's a huge maturity gain. Well for most people some stay the same or get worse. If I had to pick something though, I'd say financial responsibility.