My ex worked on a study that involved testing people's short-term memory. She mentioned a clear and abrupt change around the age of 45, though that wasn't what they studied and of course was not included in the study results. This issue is highly politically charged.

That's right when your eyes change, too. It's so predictable, optometrists can tell you what age you are.

I heard that was related to memory strategy (i.e. the brain focusing on more important methods and information) and not ability. I'd be curious on the results - clearly they aren't published though.

Our metabolism doesn't change in the technical sense, but in the popular sense of the word "metabolism" people absolutely slow down. At 20, missing a night's sleep wasn't even that big a deal. At 50 it is essentially impossible. I don't know what the underlying mechanisms are, but that's what people mean when they (incorrectly, it seems) say "metabolism".

Sleep quality pretty steadily declines with age - your 8 hours of sleep simply count for less than it used to, and you're actually consistently operating on decreased sleep

And that's even without factoring in children!

Out of curiosity how scientific or not of an approach did you take on the supplements. For instance I can't imagine you did double blinds on single supplements but did you start taking a bunch of stuff at once and assess later you felt ok? Tweak one thing at a time? something else?

A study https://directorsblog.nih.gov/2019/12/17/aging-research-plas... found some blood protein changes at ages 34, 60 & 78.

I think your last sentence is the crucial one. Up until 40, I could pretty much push through whatever tasks I needed to do regardless of how poor my sleep habits or diet had been. Now, as I approach 50, I really feel it on days where I haven't been taking proper care of myself.