Genetics is a big part of it but maintanance is very important.My father brushed daily,never flossed,never went to the dentist and died at 80 with all of his teeth intact except for a bridge.My mother who is also 80 takes far better care of her teeth,but has numerous crowns and several implants.I finally learned to go in every 6 months and to take care of the little things before they become big things.I inherited her teeth.I’m 55 and have several crowns,a bridge and two implants.
I’m 32 and got a top plate this year under similar circumstances. My mother also had a full set of dentures by her early 30s. But I go out without my plate in at least a couple times a month (not on purpose).
who did his bridge work? 
I’m not the biggest brusher, don’t really floss, almost never go to the dentist (recently went for the first time in 7 years), and my teeth are great. My parents are similarly blessed (now in their 50s) and my mom’s parents have all their own teeth (in their 80s and 90s now) so I focus on eating what my teeth need to stay healthy and assume I will always have them as well.
It’s genetics and nutrition, IMO, and prenatal nutrition much more than genetics. Periodontal disease and bone and tooth loss are not a ‘natural’ part of aging any more than diabetes is (the two conditions are linked in fact; as is heart disease). I know people in their 20s, 30s, and 40s whose teeth are rotting right out of their heads (including my one sister) and what they all have in common is a history of ‘bad’ teeth and a diet and lifestyle that gives them nearly none of what they need for healthy bones and mouths. Most of them floss more in a month than I probably have in my entire life.
I’ve heard it said that our senior years are a reversion back to infancy. A ‘second childhood’ so to speak. It would make sense that we lose our teeth in old age as we are not born with teeth.