Your idea ignores medical progress.

Mental deterioration due to aging is declining every year.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10235204/

Decreasing Incidence and Prevalence of Dementia Among Octogenarians: A Population-Based Study on 3 Cohorts Born 30 Years Apart

The prevalence of dementia decreased from 29.8% in 1986–87 to 21.5% in 2008–10 and 24.5% in 2015–16 among 85-year olds, and from 41.9% in 1989–90 to 28.0% in 2011–12 to 21.7% in 2018–19 among 88-year olds, and from 41.5% in 1991–92 to 37.2% in 2013–14 among 90-year olds. The decline was most accentuated among women. The incidence of dementia per 1 000 risk-years from ages 85 to 89 declined from 48.8 among those born 1901–02 to 37.9 in those born 1923–24 to 22.5 among those born 1930.

Conclusions

The prevalence and incidence of dementia decreased substantially over 3 decades among octogenarians. This might slow down the projected increase in cases of dementia expected by the increasing number of octogenarians during the following decades.