If you mean in physical way, until about 25 the body is producing more or equal cells as he is loosing in normal way. While until 25 the cell renewing ratio is changing and you are reproducing old cells slower and slower, so you are physically getting "old". As well as major brain changing, that you endure during puberty, normally is done by that age.
But that doesnt mean that your brain isnt able any longer to create new connections or to store new memory. So you can learn new languages or instruments with 40 as well. From my experiences your physical reflexes become a bit more lame, and you are getting tired more easily the older you get, but thats not the end of the world, you simply have to plan some nappies from now and then. ^^ Just as a bit more lame, doesnt mean really lame, so I still can play every computer game I like, I simply recognize that in earlier times, I sometimes was a bit faster from now and then.
Quote:
I'm 27, but I don't feel like an adult at all. I feel more like a teenager. I don't expect that to change anytime soon.
When it comes to mind I agree on that, so I still have very kiddie/teenager needs and likings. The physical age sadly slowly catches up, so when you are visiting metalconcerts and jump and bang around for 2 hours, afterwards your neck and knee joints slowly will start to tell you stuff about your physical age. ^^
And instead of going on Saturday evening on concert, drinking tons of beer, going home in the morning, deciding that sleeping now isnt worth it anymore because of new day starting already, playing videogames for some hours, and visiting afterwards your parents to eat with them Sunday lunch, it becomes more of...
Going on Sunday evening to concert, drinking moderate amounts of beer to prevent horrible headaches, leaving latest at 2:00 in the morning, because of your eyes closing, anyway how much caffeine you had, waking up at 14:00 afternoon, still feeling a bit lazy and dizzy, calling your parents, that you are sorry to have missed Lunch and that you will visit them tomorrow evening. ^^
The mentioned worsening of executive function during a midlife crisis. A midlife crisis normally is an sort of depression, and while an depression you are normally overly exhausted/tired/overwhelmed. Its totally normal, that during such a period, your executive functions will be sh***y, just like an teenagers executive function will be worse, when being exhausted/depressed. So you do get the need of a bit more "rest times", but as long as you allow your brain to get the rest and relax times it needed, it should be capable to deliver you comparable results.