Aging does reduce thickness and coloration of hair, making it less prominent. You can also lose density, as in hairs per inch. What tends to stay is overall coverage; if you’ve got a hairy back, it stays hairy, just less so.
Darker hair is more visible to begin with, and a lot of the old school actors were “tall, dark, and handsome” by preference.
Now, it’s been a long damn time since I read up on the how and why of hair changes from aging, but it amounts to the body just having less ability to repair itself and the decrease in skin thickness via collagen reduction. There’s a shit ton of detail missing from that, but I kinda stopped refreshing myself on medical info a while back because it sorta bothers me to not be able to use it the way I once could.
What you’re seeing in pop culture is partially a shift in what kind of looks can be a leading actor, and partly the cultural trend against mature looks leading to people chasing a “youthful” appearance to follow that ever shifting beauty standard by shaving or otherwise reducing their body hair.
Tom Selleck and Burt Reynolds wouldn’t be as “hot” by today’s standards. Reynolds did a centerfold shoot for cosmo where he’s laid out with nothing but a smile and a carefully placed hand, and it not only spawned a new magazine (playgirl), but it did it partially because he was utterly masculine by the standards of the time and hairy could still equal sexy to even very young people.
Today, that kind of photo would be ignored or even decried as him looking like someone’s dad, which usually gets used as a short code for older and less attractive. And that’s not excluding people who were alive and influenced by era; it isn’t exclusive to those coming to adulthood since the turn of the century.
There’s a documentary about changing mores regarding body hair and beauty. I couldn’t find the name with a quick search, but it is pretty interesting if you can find it.