My curiosity now is how much does one expect to lose
in one's 40s and 50s?
Yes, that's the ultimate question for the aging
athlete. The research using "normal" subjects (mostly sedentary,
mostly overweight, mostly poor nutrition, mostly low motivation) says to expect
a 1% decline per year in aerobic capacity (VO2max) after age 35. (Lactate
threshold and economy have not been well researched in regards to age so we’re
stuck with only one performance measure to consider.) There is very little
research on older, athletic, fit, etc subjects. The little that exists suggests
the decline for this group is more on the order of 0.5% per year after 35—about
half of the “normal” decline. I suspect it's less than that if the athlete has maintained his/her
training at a high level, which includes continuing high intensity
interval-type workouts. Unfortunately, most don't as they get older and so the
research is skewed a bit. As the baby boomers age there will be more research
done on truly older populations who are more performance-focused. Then I
believe we will find the decline to be considerably less than 0.5%.