But in your 30s, there's still time to adopt new, positive behaviors.
To help you pinpoint the changes you'll want to make, we consulted recent Quora threads on this critical decade and highlighted the most compelling responses.
Read on for the best ideas.
1. Start laughing at yourself.
"Laugh at yourself, laugh at the absurdity of life, laugh at the goodness and the badness of every situation," writes Quora user Cyndi Perlman Fink.
Being able to find humor in your own struggles may be important for your overall health and happiness. Research suggests that people who are able to laugh at themselves are generally more cheerful.
2. Start appreciating your family and friends.
In your 20s, you may be so busy getting an education and launching your career that family and friends fall by the wayside. Your 30s are a good time to get back in touch and remember how much you love spending time with them.
As for family, "if you can find a way to make a connection with them, you may find some things in common with them, more than you expect," says Robert Walker.
And as for friends, Nan Waldman says: "Nurture them. Laugh with them. Be silly too. Contribute to their survival and enjoyment of life. Take the time every week to be in touch."
3. Start keeping a journal.
"Journal your life! Your written records will entertain and endear in your future," writes Mark Crawley.
You don't necessarily have to keep a secret notebook under your bed - a digital photo album with captions works just as well.
4. Start maintaining a healthy weight.
Several Quora users noted the importance of maintaining a healthy weight in your 30s. "Keep your weight at a normal level that's good for your body," writes Fink.
To do that, it's important to stay active and eat right. A growing body of research suggests that if you're looking to lose weight, your diet makes more of a difference than your exercise habits.
5. Start saving money.
One of the most important financial habits to start practicing in your 30s is spending less than you earn. "Make it a habit to live below your means," writes David Leon. "I know way too many people who live hand to mouth at 50 due to excessive spending at 30."
Bonus: If you start saving for retirement early, you'll reap the amazing benefits of compound interest.
6. Start chasing a big goal.
Up till now, you may have only dreamed about starting a family, buying a house, or earning an advanced degree. Now's the time to go for it.
"It's easy to put things off," says Bill Karwin. "But it's really true that time starts accelerating as you enter your 30s, and it keeps accelerating. The time that you'll get around to those dreams should be now."
7. Start learning from your mistakes.
You probably made plenty of mistakes in your 20s. That's OK - as long as you grow from your experiences. That requires accepting responsibility for what went wrong and understanding why it went wrong.
After all, as Amanda Frisk says, "everything in life is a way for us to learn, to be better."