I think most people kind of feel a little odd about it. I thought I was a grownup when I turned 18, and then really thought I was a grownup when I turned 20. But I knew, really deep down, that it was 30 that was going to signal a sort of adulthood that would be concrete and finite. If you don't have your shit together by then, it starts to suggest something about you just ain't right.
And by "having your shit together", it could mean a variety of things. For example, if you haven't completed a college program or university degree or apprenticeship and are still taking various programs and classes that you ultimately drop out because you can't figure out what you want to be when you grow up, even though you are a grownup, and you're not holding any sort of a job and you're 30, people might start to think you're a fuck up. If you're 23, not so much.
If you have no savings of any kind and are in debt from unnecessary commercial nonsense with nothing to show for it and you have no vehicle or real estate to call your own and likely won't for some time due only to wild irresponsibility and you can't afford to live even in a crummy location without a roommate and you're 30, people might think you're a fuck up. If you're 25, not so much.
Of course, if you have completed your education, you're probably in debt from that if nothing else. And buying a house is ridiculously difficult in big cities where the costs are nauseating and not in proportion to the average couple's income. Hence why being considered a real grownup in the 20s isn't so realistic. My generation wasn't really set up to succeed in the early 20s, what with the obscene tuition hikes, lack of good available jobs, and such.
For the baby boomers, and those born thereabouts, being in your 20s was a time of being a real adult. You graduated high school and you could find a good job with your diploma and stay there. Or you could go to university, paid for from your summer job, graduate without debt into a booming economy, find a good job and stay there. You could afford to get married, buy a house (and banks only looked at the husband's income, and houses were so affordable that this was not a hindrance in the long run, though grossly unfair, but that's another story) and start a family all well before you turned 30.
Now? Forget it. The economy is lousy, education puts you in debt, houses are no being sold for two years worth of one person's yearly income. They're going for five years worth of two people's yearly incomes, or at least here in Toronto they are. So being a proper adult at 25 is pretty much not happening. Maybe in 1975 that was possible, but not now.
Which makes coming of age at 30 all the more insane feeling. It's a delayed adulthood from the early 20s to 30 on the nose, but economy or no economy, 30 is grownup town, whether you like it or not. And your brain knows it. Bodily, you are an adult, both physically and mentally. Your brain has fully developed by the time you're 25 (the frontal lobe is the last step in completion), and unsurprisingly, that's when the average person really starts to think about the future.
So, hello, 29. And a more distant wave to 30, which I'll be seeing soon. I feel moderately ready.