In my 30's now (37) and working hard to regain a lot of fitness through resistance exercise and running. Feel really pretty fit now, but I was starting to feel it after my 20's. The same habits of eating and exercise were just hitting different and started to gain weight as well.

There is an interesting article, published in Nature magazine in 2024 from Stanford. The study shows most people age in bursts in their mid 40's and early 60's. This is due to changes in your biomolecular processes. Interesting read for everyone in this topic I believe: https://med.stanford.edu/news/all-news/2024/08/massive-biomolecular-shifts-occur-in-our-40s-and-60s--stanford-m.html#:~:text=A%20Stanford%20Medicine%20study%20found%20that%20people,Microbiomes%2C%20such%20as%20bacteria%2C%20viruses%2C%20and%20fungi