Yeah the thing is, when you're a kid, there are often a lot of people going out of their way to make things fun for you: parents, other family members, your friends' parents/families, adults at school and at community places like libraries...and that's on top of your friends your own age, and the general fact that as a kid having fun is one of your top priorities.
And then if you go to college, there aren't quite so many people making things fun for you--although there are RAs, and the student life office, and various clubs and organizations doing activities, etc.--but you're in an environment where your friends are nearby and you all have similar schedules and responsibilities, so a lot fun just kind of arises spontaneously.
But once you're out on your own, in the workforce and whatnot, all that just drops of a cliff. Planned activities for adults exist, but you generally have to seek them out, rather than having them relentlessly advertised to you and/or taking place in locations where you will bump into them on the way to breakfast. And your friends are all spread out, and everyone's busy at different times, so you aren't just bumping into people and getting sucked into whatever adventure they have going on, you have to make arrangements.
It's a big shift! If you have kids, you generally figure out pretty quick that your role is now the Planner Of Fun, but without that big obvious signpost, it can feel like the world just gradually stopped having fun things in it.
But it hasn't; you just have to plan and seek out opportunities for fun yourself, because it's no longer anyone else's job to put them in front of you--it's yours!
TL:DR, when you're an adult you've got to take yourself to the aquarium and pick out a plushie in the gift shop and pay for it with your own credit card; you don't just wake up one day and find out that's happening. (Or that it's a have-dinner-on-a-picnic-blanket-in-the-living-room day, or that you're going to your cousins' house where you can splash around in the creek and look for frogs, or that's it's the day when we put the speakers in the window and let the music echo in the alley down below while we wash the car, or anything!)
