If it makes you happy, then go for it! I do think it would kind of bite for the younger spouse, though. Health concerns and random incidents can happen, but chances are the older one will die long before the younger one. There's also the fact that the older one will probably be running out of energy at a time when the younger one still has loads of it. If they're willing to live with it, though, it's obviously fine.
For me, though, the idea of being with someone who could have been playmates with my dad when he was a kid really grosses me out. Unless it was Viggo Mortensen. He can creep me out all he wants.