IF we assume that they reach 100 miles a week AFTER a number of years of gradual build up; then things are quite different (as opposed to crazily upping the mileage).
I'd imagine GIVEN also quality coaching, that most people that age could handle an average of 14 miles a day (using hard-easy days and weeks). Especially if, for non-seasoned veterans, the majority of the "speed-work" is on up-hills where it is almost impossible to speed yourself into an injury.
Myself I averaged:
Year 1: 3 miles a day
Year 2: 6 miles a day
Year 3: 10 miles a day
Year 4: 20 miles a day
Year 5: 28+ miles a day (10,000 miles).
Following 31 years: 20 miles a day (with 3 more 10,000 mile years, including as a 62 year old).
Injuries?
Early in my 4th year, my arches start spasming to where I couldn't walk. (I have super flat-feet). Got some arch-supports = end of problem. Only other injuries ever (in 36 years of Long Medium-Paced Distance: injuries due to getting hurt running on uneven surfaces or due to unseen potholes. Zero over-use injuries.
I'm NOT saying that many people should try for mega-distance training; or that most should even try 100 mpw; but I'm highly confident most 20-40 year olds could handle 100 without injuries if done with sufficient care.