I have a very clear definition of what an adult is.
To be an adult, you need to understand when it is appropriate to be childish.
I am a 36 year old man, I work in IT, I own my apartment, I pay my bills, I save money every month, I am on top of my morgage, I do my taxes (to be fair, this is extremely quick and easy in Sweden), I cook my food, I clean my apartment, I take out the trash, I do the dishes (by hand, I don't have a dishwasher), I wash my clothes and more.
I do all of that, but when it is appropriate I'll also bring out my LEGO, and my stuffed animals from when I grew up will always stay on the chest of drawers next to my bed.
I play computer games every day, but the rest of my tasks still get done.
I know when it is appropriate to be childish, and will not deny myself some fun just because other's think it is stupid.