I would say for me it was the late 30’s I just felt there was a certain energy level that I couldn’t maintain, certain things it would take longer to recover from. Also, mentally I think I was a little past my prime. Socially, my first thought when meeting or being around people started to always be, "Is this person going to annoy me - oh god it ties so much effort to be polite all the time . . . "
Any thoughts? What does the research/science say?
I think 19 or 20 is when physical decline starts. It’s just that the initial decline is very slow, and the experience you develop in your 20s more than makes up for the physical decline. I think that’s why most athletes in sports like football and basketball are at the top of their game in their late 20s.
That’s interesting, as far as the brain goes, the prefrontal cortex does not finish developing until around 25 AFAIK; so it makes sense that an athlete overall would be at his or her peak in their late 20’s.
Eyesight started getting worse in my 40s. By age 50 I needed glasses. Started having arthritis problems in my hands around age 60. Mental acuity is still pretty good. I think it’s a matter of genetics in large part.
In the last year or so I have noticed my eye sight get worse and I have been tending to bed earlier than I used to so I guess for me it is the early 40s.
I suppose a slow gradual decline started in my 30s, however at age 50 I fell off a cliff.
When it comes to mental age, I’m not sure that there is an age that everyone starts to decline. I think that’s a lot more variable.
Of course it’s also possible that someone who was a couch potato at age 20 might be in better shape physically at age 40 with the proper exercise and dietary regimen. At least this 38 year old hopes so 
The vast majority of the time it doesn’t matter if your peak performance has declined–because most people seldom operate at a level approaching peak performance. When I was a little kid I used to run around a lot. Now about the only time I even try to speed up is when I am caught outside in a sudden thunderstorm…
Physical decline probably starts around age 19 and just keeps going until you die. However in your 20s things are normally going so well you barely notice but I have known people (myself included) who had limitations in their 20s that they didn’t have in their teens.
I don’t know when the decline becomes so intense you can barely ignore it. I would guess 40s, but who knows.
According to Bill James, baseball players peak at age 27. But that is where physical decline crosses over maturity and experience, so the pure physical decline starts earlier. Mental decline is slower. In my case, I have continued to think about math and even publish into my 70s, but I simply lack the energy to really pursue ideas to the limit.
I was diagnosed with rheumatoid arthritis in my mid-30s, so obviously that changed how well I felt on a daily basis, but I was about 45 when I first noticed a measurable decline in performance not just during a flare up but every day. I noticed I looked tired in the morning, as though I needed some extra sleep. But when I tried extra sleep and still looked just as tired, I realized what I was seeing was age, not exhaustion.
That was the first point I can remember changing my routines because they had become a bit too much for me. 45 was the age when I started scaling my level of physical exertion down.
I started feeling my age very soon after I turned 50.
I think 19 is too young for many people. Bodies are still growing and maturing into the mid-20s. You may heal a little faster at the younger ages but your physical ability continues improve for a while. Bones continue to grow, muscle continues to grow, the cardio-vascular system improves. It takes time following adolescence after growth has slowed to reach full capacity.
According to this plot (which I first presented in [this post(http://boards.straightdope.com/sdmb/showpost.php?p=17146761&postcount=11[/url)), things start physically going downhill in your early 30’s; that’s when the cohort death rate starts to creep upward.
At 45, I’m getting to be more and more like this.
The bard of the '60s, Bob Dylan, once said, “Those not busy being born are busy dying.”
It really depends
Everything was fine until I hit 41.
Even 40 was okay - jumped off The Stratosphere for my 40th birthday and felt like a million bucks - but one year later I could feel the decline.
I’m turning 42 in 4 days and just started trying to get in shape again. I have developed a distressing muffin-top on my normally very thin frame that I can only blame on laziness.
I’m feeling it physically and mentally. Everything is an effort.
Actually, my back problems started at age 34. But the times it went out were few and far between, and I felt like a young buck the other times.
First shoulder surgery at 50 though. 
Next year. Always, next year. 
