.
When I was young, everyone's grandparents had dentures. The familiar plastic storage containers, which they kept by the bathroom and kitchen sinks, took up a good 4 feet of drugstore shelf space. Denture cleaners and adhesives advertised on prime-time TV. Probably many if not most elders of the time had lost their teeth during middle age and needed the dentures in their 50s, or maybe even in their 30s or 40s.
Oral hygiene habits have been the norm for decades now, so only about 10 percent of those 60 and over are without any of their teeth. Still, some people for whatever reasons have no luck with keeping their teeth, no matter what they do. They are probably a small minority compared to those who lose teeth, even in this day and age, due to poor care.
Whether for blameworthy reasons or not, loss of teeth at any age probably justifies dentures.
Anything under 70.
Never too young. People lose their teeth for many different reasons. Finances can help decide on dentures rather than $40,000 of implants.