I imagine that it's people who push themselves physically for years, either recreationally, training, competition, or for their job, and have accumulated injuries or are just somewhat "beat up" in ways that don't fully heal, but result in some arthritis, mobility restrictions, joints in need of replacement, etc.
I was in the military for 20 years but as an Air Force electronics technician I definitely do
not consider myself to be a "hard living" type

(and anyone in another branch of the US military will surely agree

... although the Air Force
does have some special ops types that would qualify!)
I think by the time someone gets to age 40 or so and starts feeling the results of their "hard living", they will know if they are one.
And, as often mentioned, kettlebell training is a great alternative for these people. It tends to provide resistance training that stimulates healthy adaptations without incurring additional cost upon their lingering ailments. (Yes that is a purely subjective statement and I have no proof... but I have heard many people attest to it.)