But we do get older, and that's what's being counted. Age, not maturity.
Also learn to crop!

I still don't even understand what "growing up" is supposed to mean.

All my time alive, so far as I can tell, it just means actively ceasing the things you enjoy and actively deciding to become a boring person with a boring perspective, imagination, and lifestyle. It's like deciding to check out and do things made up by stangers from before you were born. It also seems really bad for your physical and mental health.

Because the unstoppable march of time stops for no man. Women maybe

It means you learn to respect others and stop judging people based on race or origin (among other things)

I love how this implies men halt aging once they adults.

I don't think that's it, because I had that as a child. I just don't want to be boring and and serious all the time.

It's a mixture of things surrounding self-control which varies from place to place. I think good traits which show 'growing up' are taking initiative with taking on responsibilities, engaging in forethought before speaking and acting, engaging both fiscal and physical self-control, having good adaptability and emotional regulation, and being observant and conscientious for those around you.

So essentially just being a fairly likeable person who doesn't cause problems for others. Despite these skills being fairly straightforward and things you'd expect people to naturally gain, I think we've all met folks who don't do anything unless you tell them to, seem to give no forethought before doing or saying things, are pretty obtuse to situational changes, Have very little self-regulation, and are generally unobservant and disruptive. That goes for everyone, not just men, though there is an unfortunate amount of vocal men like this which i think makes this issue much more visible as a 'man thing'.

Also want to point out that this stuff isn't natural despite how it seems: you need good role models to set that example in order for you to follow it. There are also many folks dealing with personal traits that make developing these soft skills harder - insurmountable without support even.

I'll take any excuse for a party

I always tell people, the only problems with societies comes down to people not respecting each other, and many people not respecting themselves. Maybe the former is what part of the term "growing up" means, but it's not really a way to describe it. Because the latter is usually the result of forcing or comparing "growing up" onto oneself.

Really, "growing up" is either subjective or stereotypical. Either way, it's ambiguous and doesn't actually make any sense.

Maybe that's what Blink 182 meant all those decades ago.

Yeah we definitely need to keep treating men worse and disallowing any forms of whimsy in their lives. That's been working so well for everyone