I'm currently 33 years old and although I'm not wise or super experienced, I have learned a lot from my mistakes and from well, as cliche as it sounds "growing up". Here is my take on what you learn after you turn 30.
1. Learn to say NO. Saying no is harder than you think and depending on the situation you can be called out as a "bitch" or "selfish" or "mean" but honestly you need to learn how to say no. Now there's many ways on saying no and I found the best way is to give a short and truthful explanation with the reply of "no". That way you are honest with the other person and giving a reason why you're saying no. This applies to saying: No at work, No to a bff, No to a family member, or No to an overbooked social calendar. Saying "No" is NOT about harming anyone or being offensive but saying this instead: "I have my limit, I know what I want, and I had experienced this before so I know what to expect". Try saying no to small things and work your way up, and if someone is offended it's fine. They will get over it. This is about what you can do for that person/situation and sticking to your guns, simple as that.
2. Jealousy and how to deal with it. I am an avid reader of Cupcakes and Cashmere and she had a post about turning 30 and dealing with jealousy. It was a fantastic one paragraph insight on what she thinks and why, and I completely agree with her. As you get older, the "youngins" will come up the ladder and that is completely normal and the way of life; whether this is at work or in relationships/dating, or at parties/events, etc. There will ALWAYS be someone better than you in whatever you hone your skill in/take pride in. Someone will always be prettier, taller, better skin, hotter body, well dressed, more knowledgeable in sports or fashion, smarter, more tech savvy, etc. The best thing is to accept it graciously and move on. Don't fight it, don't challenge it/him/her, don't drive yourself crazy with it in other words. And you know what the best part is? YOU now have experience b/c you're in your 30s or 40s or 50s you can teach someone or take someone under your wing. Ta-da! But only do this with someone you do care about or want to help NOT out of spite/some weird competition/etc because things can turn ugly. Also, focus more on what is important to you and what makes your happy. When you are focused, positive, and happy you are NOT in an ugly cycle of jealousy. Trust me. So take a moment back and reflect on your own life and if you want to make improvements than go ahead and do that. Jealousy is just a feeling and remember only YOU can control it.
3. If you're not happy with your life, CHANGE IT NOW. Sounds obvious right? But nope they're people out there that suffer in silence or too lazy or in denial or whatever it is. I'll tell you another thing too; YOU'RE NOT GETTING YOUNGER. I know mean right, but that's the truth. Plus there's a thing called, resentment. And that's the worst, that and regret. Of course you cannot change ALL things like grow 6 more inches or make 100k extra or anything like that, but if you have the power and knowledge to change something in your life then do it. Work on it little by little, that's okay. Make small changes, that's fine too. Try and try again, till you get the hang of it. Don't like where you are/how you're life is; sit down and make a plan and move with it. Don't just talk the talk because no one can do it for you. ONLY you can do it for yourself. Because if you don't make any changes and just suffer inside you'll resent anyone around you, especially your loved ones. And then you get older and what else do you feel besides the obvious regret, you feel and become bitter. Trust me, I know people like this. So again if you're sincerely not happy with your life and you know in your heart you CAN do something, than do it. Yes, it's scary. Yes, you'll have to come out of your comfort zone, Yes, you will/can fail, Yes it's not easy but trust me, you'll feel much better in the end that you tried and did it than to have no tried at all.