Yes, Demi Moore was too old for her ex, Ashton Kutcher, because the age difference was extreme. When people with that big of an age gap get together, they have too many cultural differences. They also have differences with the pace of aging as they get old. One person wants to settle down while the other still has energy.
Of course women are beautiful at any age but they can be insecure at any age, too. Demi Moore was probably very flattered by Ashton Kutcher marrying her. But she should have realized that any age gap of more than a decade or so (on either side, but most probably with the man younger) is probably going to cause problems as people age.
Age is but a number, and one that is looked at far too often in relationships. Once humans reach a certain age, age should not be a deciding factor in why two people can or can not be together. The fact of the matter is that they were too different, not too far apart in age.
She was still looking good and clearly he decided that they were a good fit, so where's the harm? It certainly wasn't a typical relationship but when you're celebrities odds are it won't be one anyway, it's really a moot point and it never impacted anybody but the two of them anyway.
Demi Moore was not too old for Ashton Kutcher necessarily. Apologies for appropriating the words of a now accused child molester, but there is some truth to the saying "the heart wants what it wants". The relationship fell apart because of Kutcher's lack of maturity, meaning that Demi was not in the wrong, but that it was he that was too young.