Personally I have found my 40s to be the best part of my life to date. Way more financial security, comfortable in my own skin, smarter decision making due to maturity and just more experience. Also I think it's a myth that people can't PR in their late 30s or even in their 40s even at the elite level. Certainly if someone is a sprinter it will be hard to maintain a PR north of 35 though with improvements in shoes it is very possible.
Al Oerter set a new PR each and every time he won his 4 Olympic Gold medals over a 12 year span and set his all-time PR at age 43, 24 years after his first Olympic Gold medal and 18 years after he first broke the world record.
Bernard Lagat set his 3000M PR and an American record at age 35
Lagat set his PR and American Record in the 5,000M at age 36
Lagat set his PR and Master's (A40) world record in 10K road race at age 40
Lagat set his PR and World Masters (A40) record in the 10,000M at age 41
Lagat set an American A40 Masters Record in Half Marathon and PR at age 43
Lagat also set his PR for 1500M and the mile after age 35
Now these are just two examples of elite athletes setting PRs into their 40s and for certain events such as the 100M it might be harder but Kim Collins hit his PR of 9.93 at age 40, thirteen years after he was the 100M world champion.
Troy Douglas Current Masters A40 record holder for the 200M set his PRs for both the 100M and 200M at age 39
With training and advances in nutrition and equipment setting PRs into one's 40s is possibly not only for elite athletes but even more so for those who are casual runners. Personally I'm running much faster in my early 40s than I was at 18 because of more disciplined training, better sleep habits and diet.