Hard to say. Plenty of metabolically damaged homeless, too.
If I had to guess...
Stress relatively low, good light environment, cold exposure at night, sugar in alcohol, agressive environment.
Discussion
Oh yes, they are definitely metabolically damaged.
Stress is not necessarily low in that group, and if it is, it's because they are constantly drunk. But alcohol is a stressor. Maybe the environment. Probably something related to exposure to certain microorganisms or traumas at specific ages in life.
I've been watching this pattern since 2013 (homeless and otherwise) and the metabolism-hair theory does not match my observations.