I think they're harder because the two people are clearly at different points of their careers or lives, which can be tough even when the people are the exact same age.
My co-worker was basically the same age as his wife, they wanted to retire together... but he was ready to retire NOW, she wanted to wait a few more years... so he did, but as the years went on, he was ready to retire and she still wasn't. And he ended up retiring, I don't know about her.
But it's more about people, though I think they can work, just a lot less likely to work in the long run.
But hey, over 50% of marriages in the USA have ended in divorce, so one could claim long term ain't working anyways...
