Hey M. :) we're pretty similar: I'm almost 39 and I have 5 kids, my oldest is my daughter who's 20, then I have an 18 year old boy, a 17 year old boy, a 2 year old girl, and a 3 month old baby girl... so I have all ages! :) It's hard when they're 20, they do think they know everything sometimes!
What I find with my 20 year old, though, is the more I talk to HER, the more she'll tell ME. Sometimes I'm thinking she's sad about her boyfriend, but she won't tell me, so I take the opportunity to tell her a little story about myself, something I went through that I think she might relate to. This makes her feel comfortable, and she'll often say "well, ya know, Jeff's kind of that way too..." and tell me something her boyfriend's been doing to her, making her feel inadequate or stupid. I figure, if she knows I've been there, she'll know I might sympathize with her, so I am a good person to talk to about such things.
I'm pretty honest with my kids, and they seem to feel free to talk to me about any topic, because 1.) I don't overreact, I stay calm even if I'm freaking out inside! and 2.) because I tell them stories, they know I've 'been there', so I can relate, and have good advice. My kids know what stupid things I've done in my life. I don't just tell them what dumb things I did, I make SURE to tell them the outcomes and consequences of my actions. I don't say "yeah, i used to smoke pot too", I say "yeah, I smoked pot when I should have been doing my homework. All my friends graduated, I dropped out. How hard it was going back to school in my twenties!" and so on. This way, they know I'm not perfect, but they know I"m honest with them, they can be honest with me. And my advice isn't some advice some counselor is giving them in a school. Kids are smart. They know when a counselor is giving them advice, in most cases, that counselor is just telling them something they read in a book somewhere, they don't have personal experience. If you have personal experiences, share them with your kids. But mostly, share your consequences, things you have learned as a result of going through difficult times. That's the best way to open the door to communication with kids that age! And... once you've talked, make sure to give them time to talk. Ask them questions about how they FEEL about things. Try not to sound like you're interrogating, though, cause that'll shut them right down. Be careful never to bring up things they told you in confidence when you're mad and throw it in their face! they have to know their secret is safe with you. You can say things like "where would you like to be when you're 30? How would you feel if.....?" that kinda thing, that opens the doors to them talking to you. All my kids freely talk to me about every aspect of life, it's incredible. I have the relationship with my kids that I always dreamed of having with my own mother. My kids know they can freely come to me with any problem and tell me ANY thing, and I will calmly help them solve whatever it is. You can have that, too, believe me, this works. Good luck!