When I was younger, my age and corresponding lack of success- according to what I wanted to achieve by when, really troubled me. My twenties were especially hard. Age 28 in paticular. I remember some of my teachers were around that age when I was in secondary school and I'd got it into my head that that was the age I needed to have my shit together by. As it was, by that age, I was stuck in retail job I hated and decided I needed to start again with a second degree to get me back on track.
I think, as I aged, the idea of age became less important. I realised that plenty of people find themselves treading water in life and find they need to change course. It would be nice if we had the benefit of hindsight when we have to make major decisions but, it doesn't work like that.
In terms of maturity, I agree with you. I think it's more about the experiences you have had in life but also, attitude. I know people in their 70's who are still doing very physical, hard jobs. Most people are complaining about their joints by that age but, they power on through because they love their work. So much is to do with attitude I think.
At the other end of the scale, youngsters vary enormously. Maybe it comes down to what was expected of them to a degree. Someone who was either neglected or, given encouragement to do things on their own may seem more mature and better at 'adulting' than someone who was 'babied' and not given any responsibility.
Plus, obviously experiences matter. Especially bad ones that create trauma. It seems to affect people in different ways though. I think it certainly does kind of age someone though. I think I became very cynical very early on because of a fairly rocky childhood. It can work the other way though too I think. Certain traumas maybe trap us in a more immature state. I certainly have anxieties that feel too frightening to confront and heal. Social anxiety in particular. That isn't a very mature attitude. A 'proper' adult would probably be strong enough to both recognise the things that held them back and, deal with them.