70 years old should have limit on miles per day.

There are some 25 year-olds out there that shouldn't drive more than 20 miles a day! At the opposite end of the scale, a transit driver at the agency I worked at was still putting in a full 8-hour day when he eventually retired as he approached his 80th birthday.

I my case (at 71), I get serious leg cramps at about 150 miles, so I tend to keep below 250 a day, with the last 75 being done with increasingly frequent stops for stretching.