I'm in my early 20s and the man I like is in his mid 20s. While the age difference is pretty nil, the maturity difference is the width of your typical ocean. Whatever that width may be. Okay, bad metaphor.
.... first maturity test? haha.
When he and I first met, I felt like his daughter instead of his date. Despite being an English major, his vocabulary was double mine. He appeared in a suit and I - without thinking much upon it - showed up in a long sleeve shirt and booty shorts. Whoops. But I realized with time, that this severe difference in maturity was exactly what I needed.
My first recommendation, don't be intimidated. Okay, that's impossible. You're going to be a bit unnerved when he starts talking about his position in a major financial corporate, and the only thing you can contribute is: "Umm well I've taken a Finance class in uni before!" Recognize that he's been in your place before - he was once as immature and/or inexperienced as you. Everything you find embarrassing or awkward about yourself, he can likely empathize. And because of this, he'll have no issues communicating with you. He may even find your bumbling cute or endearing. He's experienced. Take a risk, and blurt out that silly joke. He'll know how to handle you.
Start dressing better! He may forgive your booty shorts the first time, but he'll eventually be bothered by your dress code. He wants a woman, not a girl. Throw out those Pokemon underwear and invest in some big girl panties. Oh, and heels. Don't forget the heels.
Acknowledge the benefits AND detriments of your crazy hormones. No more p*rn marathons, time to pay some sexual attention to your significant other. If you can't give up the p*rn (and you shouldn't have to), ask him to watch it with you. He may enjoy your insane sex drive but remember that you can't pounce on him at every second of the day.
Less drama. Possibly the biggest difference between an older dude and a younger one. They expect less drama from you, and will give you less in return. They don't want to hear about so-and-so cheating on her bf with so-and-so's mother. Keep that to yourself and your gossip queens.
You should also realize that hanging out with an older, more experienced man will benefit your own personality. He'll constantly challenge you to improve yourself indirectly or up front - you'll have to exert more effort and brainpower to sustain a conversation with him. It'll be a huge contrast to the "swagaholic" friends you may or may not have grown up with. Sooner than later, you'll develop as well. You'll become more alert, professional. You'll mature.