You're supposed to dive within your abilities. It could be a grossly over-weight diver, an old diver with limiting strength who can't get up or down the ladder with tank on their back without help, let em dive, but make sure the dive sites and the dive conditions are tame enough to give them plenty of margin for error, cause sooner or later the weather will turn bad and conditions will dramatically deteriorate while you're on your surface inter val and you'll press your luck, or a dive site's conditions will change, or a dive master will miss read a dive sites conditions miscalculating a tidal current's time to change direction, there will be a sudden down current, the dive boat will catch on fire, run out of fuel, break down or have to leave you in the water for any number of reasons, or one of a thousand other things will eventually come along and then those diver's limitations are going to be a liability, how big and how fatal the out-come will be up to fate and chance.
I personally can think of many dives I've done in the past that got hairy that thinking back I wouldn't do them again if I suddenly acquired some physical limitations to my body.
Just a little over a year or so ago I did a shore dive in Maui that I got out of with just some scratches and bruises, diving with sketchy poor deteriorating weather conditions combined with a navigation mistake made me miss the proper exit on the return and had to haul up on iron shore with waves smashing me into the sharp lava and urchins, adding insult to injury I happened to have left my booties in the car and went in barefooted in my fins (was a sandy beach entry and planned exit) but lava, bare feet, urchins, pounding surf, was ugly and embarrassing but survivable with mostly my ego more bruised than my body. Had I been physically handicapped in some way it would have ended way, way, way worse.
**** happens, I'd like to have some physical margin of error on my side for when it does, cause the more you dive the more likely something is going to happen out of the ordinary. If you're on the razor's edge of margin you're going to end up an accident thread on scubaboard.
The old diver with the physical limitations may have read the weather better or not made the navigation mistake or just looked at that location and said there were too many things that could go wrong. Or they might just have a heart attack and die.
When you start drawing lines based on physical abilities you need to be careful. You may end up on the wrong side of the line. The vast majority of divers I see are old people, over 50, many over 60, lots over 70. And I dive mostly in the Pacific Northwest in conditions that are a little tough. I don't think the dive industry would survive taking us out of the buying pool.