Unless you're the child of a teen pregnancy or a late surprise, you are probably going to be in your 30s when you finally have to deal with your parents rapidly transiting from "respected older adult" to "elderly person" in their early 60s with all the associated health problems. If you have siblings close in age to you, you may also need to deal with differing opinions about how much help your parents need or how much you even want to help them.
Some of your friends will get married or have kids in their 30s, and will then often not hang out anymore with any of their friends who arent married or don't have kids. If you want to date someone your age or older you will find that an increasing number of them are either divorced or have children from a previous relationship. You can also date younger people, though if you go below 25 you'll probably be shocked at how much less mature they are than you remember being at that age.
At some point, if you dont get married and have children in your 30s, you are probably going to find yourself living alone in a city where you don't know anyone. You will need to figure out how to go from having few acquaintances and no close friends nearby to having many acquaintances and a few close friends.
Some of your friends will join a cult in their 30s and disappear from your life, because they couldn't handle how lonely they were. And it won't be the fun kind of cult either, but the depressing sort of self-help psychotherapy high control group that preys on insecurity and reinforces it. At some point they may want to reconnect, and you'll need to decide whether they're deprogrammed enough yet to be safe to interact with.
In your 30s you'll need to eat properly and exercise more, because the older you get the more you'll pay for not doing so, and you're finally the age when this will start to catch up to you. You might actually even need to go to the doctor once in a while, but it will always still feel like a waste of time, even if they do something other than tell you to eat properly and exercise more. Most tests are routine and will be negative, and no one involved will respect your time, but you will not regret catching something like high blood pressure or thyroid issues or even skin cancer earlier rather than later.
Some of your friends will get into crossfit and alternative medicine in their 30s because they don't like going to the doctor. Some of your friends will neglect their health entirely in their 30s and stay out partying too late like theyre still 22. In both cases, you'll need to decide how much of their increasing levels of bullshit you're willing to tolerate.
In your 30s you'll constantly find out about new things that you were apparently supposed to have done in your 20s. It's not possible for any one person to have done most of it, and if you dont learn to ignore these arbitrary expectations and milestones, you're never going to feel like a "real" adult, no matter how many younger people there are in your life who look up to you and respect you.
Some of your friends will go back to school to get masters' degrees in their 30s. Don't bother unless it's going to make you more money and change the type of job you have to something you'll enjoy more than whatever you're doing right now. Most of them will not finish and regret it.
Some of your friends will be trapped in bad marriages or be forced to move back in with their parents or be underwater in their mortgages. it's relatively easy to end up in a situation in your 30s where you feel trapped and it's hard to extricate yourself, and easy to see someone else in that kind of situation and get sucked into their pit of misery trying to pull them out of it. regrettably in most cases if they could get out with your help they could get out without it. dont jump in the water to save a drowning person, you'll both drown.
Also, on a lighter note, you'll have no damn clue what the kids are into, even if you have kids pf your own. fortunately you probably wont care, either. and you'll think the 20-somethings are weird for caring about it. most of the positives of being in your 30s are not very good plot points, because it's mostly about being able to not care about things that dont matter and about being able to handle problems that arise with ease that gutpunched you in your 20s.
