Do couples with huge age gaps go the distance?

I have seen very many couples go the distance with a decade between them but not two decades. There's something uncomfortable about seeing a grossly mismatched couple. Others hesitate, not knowing whether to ask if it's a daughter rather than a wife, a grandmother rather than a mother.

For one thing, there's a very high certainty that the much older partner will die well before the other, which suggests that the younger is completely prepared to start again. It doesn't suggest full commitment, somehow. Always best to keep broadly within your own generation, at least.