When I announced my engagement on Facebook almost six years ago, an old friend who had been married since he/she was in his/her early twenties congratulated me and said that I'd be spared a lot of suffering, getting married so late.
I am sure he/she also said something like "It's been great, but..." because he/she has a lot of kids, all of whom are wonderful and gifts from God, but...
The longer you are not in a fully sexual relationship, the fewer kids you are likely to conceive. That's just how it is. Any Christian who is really, really certain she does not have what it takes to raise children should not get married until she does. Natural Family Planning is a great tool for actually implementing some kind of family creation plan, but a good rule of thumb is that if you are under thirty and having frequent sexual relations, without contraception, you are going to get pregnant.
I'm not sure, though, why a Christian woman would be so sure she does not have what it takes to raise children. I suppose that if she has NO experience of caring for children, or pets, or plants or anything, then the idea of being a body slave to an extremely intelligent yet vulnerable human being must indeed be scary. However, women--most of us dirt poor--have always had and cared for babies. The vast majority of women in history have not worried about their babies' second language acquisition or whatever worry is the current obsession these days. The vast majority of women simply hoped (and hope) to keep their babies alive. If you can keep a baby alive and as comfortable and clean as possible, then you are good mother material.
But although babies and how many you might have, and how closely together, is uppermost on the minds of young Catholic women who hope to marry one day (and do boys think about the baby question quite so much?), there are the less pleasant spectres of abuse and divorce. When I was 21, I taught Latin to a class of twelve and thirteen year olds, and one day they were fascinated by the concept of annulments. This school, incidentally, was so anti-divorce that it asked me during my job interview (totally against the law, by the way), if I were divorced. It simply loathed divorce. And so my little moppets were fascinated by the topic of annulments which, I believe, they saw as a get-out-of-jail free card they would keep JUST IN CASE. Little did they or I know at the time that one day I would seek and receive an annulment myself.
Abuse is a tricky one, and all I can say is that an abusive 19 year old boyfriend is not going to stop being abusive once he's married, and neither is the abusive 19 year old girlfriend. The fastest and most effective way not to be abused by a spouse is to not marry an abusive person. In Canada we say (or used to say) as first date advice, "Watch how he treats the waitress, 'cause that's how he'll treat you." I have found a lot of truth in this, although there is almost so much an abusive man can do to a waitress compared to what he can do to his girlfriend, wife, sister or mother. Incidentally, any man who hates his sister or mother with a passion needs a therapist, not a wife. Even if his mother IS a real witch, unless he has made peace with that, he's going to pelt you with his mother issues.
Another way to avoid abuse is to put off marriage until you are old enough to handle carnaptious and aggressive young men. There are 19 year olds who can stop a rampaging rapscallion at ten paces, and there are 27 year olds who can't. Knowing how to look a loved one in the eye and say "Hey, you can't get away with that" is something that, for me, came with age. And I'm still not great at it, so it's just as well I married sweet-tempered man whose motto is "Anything for a quiet life."
Really, nobody should get married until she or he is a mature adult, and maturity comes at different ages. It would be better for the mature Single of thirty-plus to date a mature Single of twenty-five than an immature Single her own age. A mature 25 year old can handle marriage-and-babies, an immature thirty-something can't. He'd rather wait until he is 40, and then at 40 he may very well say he feels too old now.
The greatest disadvantage of old marriage is the risk of not having ANY children, but one advantage--if you feel this is an advantage--is that you will almost be guaranteed a smaller family than you might have had, had you married at 22. But the far greater advantage, if you have always been a late bloomer, is that you will have more confidence and maturity. You will now know who you are, and what you can and can't put up with, and the humility to change what ever it is that you do that drives your loved ones nuts. Having had many of your illusions shattered, you will be much more rooted in reality than you were when you were a wee sprog of 25.
And now I must rush off and do my Polish homework.