For all of the 50+ having the most, in general, we may just be seeing the baseline principle that time-in-the-market beats other effects.

For why they have more than their 60s and 70s counterparts, I can only guess.

My guess is the 55-64 generation has less conservative investments than their 60s and 70s counterparts, so they raise higher on a strong market, and fall further in a weak market.