Why would we do this? A spry and highly experienced 70 year old is a great option. Advances in medicine will all but ensure this is the case going forward.
Because according to research, cognitive decline typically starts between 45-55. Even if you are an outlier, it is unlikely that cognitive decline has not begun by 70. I included a link previously, but here are some additional pieces:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2683339/
There is some evidence that the magnitude of age-related decline accelerates at older ages. To illustrate, a sample of about 800 adults between 61 and 96 years of age in my laboratory had cross-sectional slopes of about -.04 to -.05 SD units per year compared to the slopes of -.02 to -.03 SD units per year for adults under age 60. In absolute units, the decline in speed variables was about twice as great in this age range compared to adults under age 60, and the decline in the memory variables was nearly 4 times greater. What is not yet known is whether these quantitatively different age trends reflect changes in the same set of influences, or the operation of qualitatively different types of influences. However, what does appear clear is that several different types of results converge on the conclusion that age-related cognitive decline begins relatively early in adulthood, and certainly before age 60 in healthy educated adults.
http://ns.umich.edu/Releases/2001/Aug01/r081301a.html
http://sphweb.bumc.bu.edu/otlt/MPH-Modules/PH/Aging/aging_and_cogntive_decline.jpg
More importantly, how does lack of sleep factor in? I, like most, noticed change in the amount of sleep I felt I needed in my 20s vs. my 30s and even the difference between Kid #1 and Kid #2 was noticeable. How does that factor in in terms of mental health for a 50 year old vs. a 70 year old?