As you see below, records decline at a relatively steady rate between the ages of 35 and 70-75, then the rate of decline picks up rapidly.
We see that distance running and swimming drop off faster than sprinting, and Fair notes that women drop off a bit faster than men. Chess declines are much slower than the physical activities.
Fair has calculated age factors like those graphed above for men running and high jumping, men and women swimming, and chess. For example, here are the age factors for short races -- 100, 200 and 400 meters -- by men:
Using the table, you can see that if, for example, you ran 100 meters in 12 seconds at age 35, Fair would predict that you would have slowed to 1.23 x 12 = 14.8 seconds by age 70. (His tables use age 35 as a reference point).
If you have a time from an age other than 35, you can compute a predicted time using the ratio of the the two age factors. For example, if you ran a race in 20 seconds at age 50, you would expect it to take 20 x 1.23 / 1.09 = 22.6 seconds at age 70.
You can download a spreadsheet with all of Fair's age factors or use his online calculators to make your own estimates: