My 20's was over a while ago, but honestly I really don't miss it that much. Sure I had a lot less responsibility, but over all I don't miss it. Here is a list of 10 things most people regret about their life when they were in their 20's.

Not traveling more
Not saving at least some money (compounding interest is awesome)
Spending money on nicer cars, apartments and restaurants when you won’t remember any of that stuff later
Getting in shape (wasting your best years being fat, tired and unhappy)
Not developing long-term friendships
Not appreciating that you may never have as much disposable income or as few responsibilities as you have now
Not prioritizing experiences
Not taking care of your teeth
Spending too much time nurturing relationships and friendships and that weren’t healthy
Letting “the one” go because you’re too young or too poor or whatever (this is only a regret if they really were “the one”)