What I found when I flew to the USA in October with United and was speaking to some of the gals down the back, was that a lot of the people of eldest seniority come from the days when you had to be single to be a hostess, and as soon as you got married you were asked to leave the company.
When these rules were uplifted many came back, but many have lived out the years of hostesses, stewardesses and now flight attendants, and never got married, so flying is pretty much their life. As long as those crew members are capable (and they must be to still be flying) perform all the service & more importantly safety features of being a flight attendant, they can keep on flying.
Another area perhaps is the turmoil over pensions at US airlines. As many airlines try to get around the huge costs of pensions for the US carriers, many flight attendants have the incentive to keep flying & keep earning!
Just my $0.02
