Why would men and women pushing into their ages be in the lower decks? The most rational and logical explanation is that the elder members of Starfleet are all Commodores and Admirals commanding fleets and star bases. I wouldn't expect to find someone in their 80s to be part of the crew of a ship in deep space.
Because the odds of becoming a ship captain are somewhat low.
90 percent of upper tier guys likely never see the captains chair.
Although I think its likely there are many people with the rank of captain not serving on ships.
Captain sisqo being a prime example.
But its likely many many more settle down on planets, and take up working on terraforming projects, shipbuilding, research assignments etc.
If you know many people who actually live on ships, you quickly understand its not a long term place to live.