What impacts the (often-gnarled) shape of people’s toenails?
And what causes this to change over time? Is it just aging, or are footwear a factor?
In my case, toenail fungus. 
Yeah, fungus.
Through proper foot and footwear hygiene, I eradicated all of the smell-producing bacteria in my shoes and socks and feet. I can pull off sweaty sneakers and take a deep whiff and not get one note of foul odor (unlike in my youth). This probably also killed off any fungi and spared me the fungal distortions my parents suffered with really thick and gnarly toenails. Mine look like they did in my youth without any weird thickening or gnarling.
Well in my case I don’t think it has anything to do with fungus.
But when I was around 20 or so, the nails on my big toe began curling down at the sides, so that I began having problems with ingrown toenails unless I left them a bit long at the corners.
And now, the nail on one of my small (pinky?) toes has begun turning up at the side.
Very weird. I would have thought whatever your nail pattern is, it would stay that way, at least in adulthood.
10 km per day run!
Fungus gets my vote. I had it and my big toe and little toe became “deformed”. The big toe building in the middle and the little toe growing straight up. Doctor gave me the Lamisil pill and after about six months, nails are back to normal. I can now go without shoes and not be embarrassed!