I would be concerned with secondary effects of aging on viability of gametes and chances of exotic conditions - doing this over any large span of generations would probably (temporarily?) yield a much higher mortality rate until we figured out the best ratio of increased parent lifespan to increased child mortality.

With humans you probably wouldn't want to wait with the breeding until the parents are old, but select breeding pairs based on the longevity of their (great-)grandparents. At least until you figure out useful biomarkers that reliably predict longevity already at the age of twenty or so.