I never saw any problem with people of different ages marrying, any more than interracial marriages or those where the people are from two different countries. It's just one more thing that can create differences. There can be a problem when an older sister who has been independent for 20 years or more marries an immature brother where he is going to make crap decisions and she will have to abide by them. This is more of a problem in the Tower, where such control is the rule.
What is a problem is where the sisters have little or no choice in the matter. If they have no one to pick from because they have to marry within the Tower, they are going to get stuck with a lemon either way. And, in many cases, the age difference is the least of their worries if the husband turns out to be a tyrant or otherwise abusive of the wife or the children. And, there is no way of justifying an older man initiating the use of force or fraud, or threat of force, to get the sister to marry. That includes having them marry underage sisters, often because the sisters are scared that they will be left without any other choices. That, not just the age difference, is what is wrong.