In your twenties....
1) Your parents or close relatives begin to hit their forties or fifties or sixties, and begin to get sick.
And they begin to become unwell, or the family finances begin to get stretched (parents nearing retirement, three kids in college and having to pay fees etc) and things begin to happen.
You're not a child anymore so you are included in adult conversations about money, health and the care of relatives. Your family members may expect you to contribute to the discussion and possibly also contribute in other ways (financially, if you've got a job etc).
Previously when you were a child you were always the first person people thought of when bad things happened, and the first priority was usually for support to be offered to you because you were 'too young to handle it'. When you hit your twenties, the shields are lifted and you'll have to be privy to the difficulties and problems hitting your family circle and instead of being the supported, you become the supporter. You may be the only person capable of driving your grandmother to hospital for appointments, etc. You may be the only person your parents will talk to about their financial troubles. And people expect you to be able to handle it. You're old enough now.
Some of your family members may pass away during this time. Some of your friends may pass away. I've had both happen. You'll have to deal with it, and sometimes be the one who takes charge when others are distraught. You'll have to not only learn how to deal emotionally as well as practically with death and loss, but also how to do this without the support that you would have had when you were a child.
2) You have to change the way you view relationships with your girlfriend/boyfriend, friends as well as your family.
Often this is a time when you start working, buying a house on your own, or a car, starting a family and so on.
When you were 18, not wanting commitment was fine. When you're 28, your girlfriend may leave you if you're not forthcoming with the ring because she wants to settle down and find some stability and you're not offering it.
When you were 18, spending entire weekends smoking pot with your friends was great. When you're 28 with student loans to pay and a job that you could lose if you show up high, hanging out with people who insist you do the things you don't want to do is just asking for it. You may have to leave a group of friends. The weed example is more drastic than others, but often the friends who drag you down do it subtly, without even realising it themselves. Every time you want to go and apply for a new job, or take a new course, they might laugh at you, or say 'but that will leave you less time to spend with us'. And eventually you'll look back a bit late and realise that you should have taken those opportunities because now you've been passed over for a promotion because you're not qualified enough etc.
When you were 18, your parents were a massive drag and you couldn't wait to get out of the house. When you're 28, your dad may know how to fix the boiler without you having to fork out money to get someone else to do it. Your mom may be able to babysit your kids. Both of them will now have great experiences that you can gain from them - they can be something like walking quoras. Of course this may not apply to everyone, but certainly since I hit my twenties I realised how much my family's support means to me.
3) When bad things happen, they hit you harder, and you bounce back less well.
When we were kids, if we ran out into the playground and fell down and grazed our knees, everyone flocked over to take care of us, and the wound would heal in a few days and it would all be forgotten, and we would be out and running again before the day was out.
Now that you're older, the worst case scenario is much more grim than when you were a kid. If you get fired from your job, you could stand to be unable to pay your rent and end up in debt. And say you do lose your job, and you have to start from scratch and do everything again to get back to where you were before, you don't forget what happened. Some people don't get to bounce back from the bottom of the pit as easily as others, and end up unhappy and negative for months or years, sometimes for the rest of their lives.
The secret to getting through it is to keep looking forward and trying hard. If you're not used to this then it will be an added challenge for you to keep convincing yourself there is a point to waking up in the morning. It's the ultimate challenge, and one that is almost prerequisite in order for you to actually become an adult. How old you are doesn't measure maturity as well as how many times bad things have happened and how you've dealt with these things each time.
Of course the question asked means I've had to talk about fairly depressing things, but i think it's important to remember that it's a time of experiment, joy and new experiences, so enjoy what you can, when you can!