I am not convinced I was at a peak 30 years ago.
But from a holistic wider view, you don't necessarily peak in your 20s as your gains in knowledge, experience and wisdom compensates for the decline in pure physical performance.
35 year old me could beat 25 year old me in almost any physical contest. Because I am in much, much better shape now. I do regular cardio and I train and lift weights. I was chubby and out of shape at 25.
I suspect the same could easily be true for mental prowess.
It's like driving 25 mph in a porsche vs 50 mph in a SUV. In this instance, the SUV is faster because you chose to go 25 mph in a porsche. You didn't fully explore the full potential of the porsch.
Your physical peak could be at 25, but you also could choose not to exploit it fully.
I am not so sure - I wonder if there are ways, in some fields, to keep getting better well into your 50s. For example, I don't really know of the ability to gain strength decreasing. So it seems like you could lift well into your 50s and end up stronger than a peak 25 year old.
I.e. an elite 25 year old wouldn't drop off in strength.
...I just answered my own question: https://torokhtiy.com/blogs/warm-body-cold-mind/weightliftin...
Seems like 35 is where Olympic weight lifters stop winning medals. There is also this: "Harvard University says the average man loses up to 5 percent of muscle mass every decade, starting at age 30; most men lose about 30 percent of muscle mass as they age."
Well, I guess I am going to have to stick with the guy who posted "older people just stop giving a " to comfort myself about getting old.
Though at 55, you are not at your intellectual peak of the your 20s, you have gained knowledge and experience that can more than compensate for loss of brain power.
I like to think of it in terms of hardware and software. While the hardware may stagnate and decline, you can always update the software to get improved performance.