Hey Precious Roy, I got excited when you mentioned Simonaitis. You must live in Utah too. You make a very good point, that it is essentially social/economical considerations that make it appear that running ability plummets after 35. I've heard from D. Cabanillas that the real drop off comes after 45. I think the opinion of people who have actually attempted to do serious training for decades would be the experts on this subject. I think it is also internal mental considerations that play a big part in the drop off. i.e. a larger part of yourself realizes that although running is fun, in the grand scheme of your life, it is a childish and fleeting pursuit. But if you find a way to balance your pursuit of running with the other pursuits/responsibilities in life, you can still train as much as is needed. If you already have years of cardio base, I think our sport really only takes 10-12 hours/week to be at 99% of your potential, any more than that and you just get tired and injured.
There are definitely changes that need to be made in training after 35. More time needs to be taken to recover (running 2x a day maybe becomes more of a liability than a benefit), and your 5k PR is probably not going to improve. But there have been training runs and races I have run after 35, where I feel stronger than ever before. Maybe that just means I didn't explore my true potential in my 20s, but I really believe that if you are flexible enough to make adjustments and listen to your body, you can still really surprise yourself in your late 30s and maybe even early 40s. I'm excited to find out.
Lagat is the perfect example to see what a 40year old can run for 10k. Based on his 1500 PR, I would say he's as talented as anyone on the planet, now we get to see what that talent can do at 40 when moving up in distance and when properly financed and motivated. Perhaps the greatest attribute working in his favor is that he has always been flexible in his training (taking time off to avoid injury). As a result, he still has the physical and mental energy to do get the most from his body.