I think the ideal age difference is not based upon levels of psychological maturity; but on other factors. The primary purpose of Marriage is the formation of a family, so given this fact:

For men, what determines the best age, is the ability to lead, protect, and financially provide for the family. Most men achieve this readiness by age 30 or so.  

For women, it is the ability to biologically bear children and nurture them, which determines the best age. Because fertility is intrinsically connected to the youth of a woman, she should marry young. 18-22 years being the ideal age, in my opinion.

Also, men and women are actually very different creatures and as such, they age very differently. I'd say that the ideal age difference is about 15 years, with 10 being the minimum, and 20 the maximum.

.
Most States don't have any requirement for marriage candidates being 18.
New Hampshire requires 13 for example.
Several states require 14, others 15, still others 16, and so on.
Liberals are pushing legislation to link age requirements with discrimination.
sɛҳuąƖ orientation laws have already been passed so now if you're opposed to a mature man having child wife, you're a bigot.
They've even got a term for someone who has a preference for young girls.
So if you don't think that's okay, you're the problem for sɛҳuąƖ discrimination.

At 20 years old, you perhaps disagree with what I wrote above (according to the OP), but you may change your opinion when you reach 40 or so.

Hmm... that means I haven't much time left to change my mind and agree with you. 
.
.
OP - Be patient enough to find a good spouse and not let a desire to be married cause you to choose poorly. Also, be patient enough to finish your educational goals or you'll likely regret it later--for some, they're past this by age 20 ... others will take much longer. It's difficult enough to meet those two requirements (mostly #1). Don't let an "ideal" age difference keep you from marrying the right person.

It's natural for the man to be older than the woman, for many reasons which can be summed up best as "human nature".

But that is as far as we should go. I disagree with the ridiculous "10 year minimum" as suggested above. Maybe that worked out for her personally so she's going to apply it universally -- but there is no logical reason to do that.

Women can't stand to marry someone they consider inferior to them. That goes for intelligence, maturity, career, income, etc. On the other hand, normal women don't seem to be too focused on looks -- or even general fitness (weight, for example).

And yes, maturity and wisdom are important factors in determining if a given young man is ready to marry. He also has to be able to provide for a family immediately after marriage. If he's still in college that's a problem. You can't "wait a few years to start a family" like pagans on birth control.

I bring up wisdom because if you're not careful, you might end up marrying poorly:
https://www.cathinfo.com/anonymous-posts-allowed/young-men-beware-marrying-novus-ordo-women/

A lot of young guys don't think enough, or they go along with the mainstream too much ("she's hot!") but aren't thinking of the future. If the young lady wasn't raised in the Catholic Faith, or with conservative values, it's not going to be part of her. She might convert and go along with a strict interpretation of the Catholic Faith while times are good, while she's "in love" (the emotion) with you, but if times get hard, disagreements arise, the honeymoon ends, crosses and challenges arise, and hard decisions must be made, that protestant, pagan, or Novus Ordo upbringing will come back roaring with a vengeance.

I appreciate the thoughtful responses. As time goes on my desire for marriage becomes less and less and I have a greater interest in the priesthood. Incidentally, is it possible for me to become a priest if I'm an "illegitimate" son (My dad divorced his [wife and "married" my mother)? I understand the Society itself doesn't allow illegitimate sons to be priests, at least I think that's the case. I wonder if it's different in the Resistance?

Another thought I had -- in the Middle Ages, the height of Christian civilisation, people weren't getting married in their 30s, were they? It was normal for people -- men and women alike -- to get married in their "teens". I really don't see anything wrong (as long as economics and morals are settled) with it.