I think there's a potential for many complications that are unrelated to one partner or the other moving. I agree with whoever said that shared goals are important. But how realistic is it to expect a teenager for example) to share dance goals with a retiree? Sure. Theoretically, it could happen. But does it, especially given the different levels of physical ability for each age group.
I don't want to sound like a smart a$$, but if they don't have the same dance goal, they shouldn't partner up at the first place. No?
Well, unless at the short run, their goals met - or at least each could offer something on the table.
No offense taken.

In an ideal world, you may be right. But sometimes you have to take what you can get, partner-wise. And sometimes, people grow and change. My point is that,all other things being equal, a teen may very well be able to achieve higher levels and better results than a middle aged partner. So, even if they both start off at, say, syllabus, there may well come a time when the teen becomes dissatisfied by being constrained by a much-older partner's physical limitations.