You don't understand mother nature, women, and love.
One of the secrets to a good love relationship is from the woman's often really, really strong romantic feelings when she is in her teens. That's just a fact, Jack. Live and learn that fact or suffer the consequences of ignorance. Some movies will say "That old zing" -- well, teen girls are awash in the propensity to "zing".
Sure, we'd all like some thoughtful, planned, rational, etc. relationship -- alas Mother Nature didn't get or like that memo. The relationship gets a LOT of that young women propensity or gets, net, less.
Bluntly, the woman is able to be good at marriage and motherhood years before a man is able to be good at marriage and being a father. That's just Mother Nature's way. In particular, the woman with a baby needs to feel that her husband is strong, wise, and cares about, cares for, and will take care of his wife and children. If he is close to her age, then that's super hard -- possible with some special circumstances but otherwise improbable.
Or, when she is 18 she is old enough; when he is 35 he is still young enough; and that's enough. Besides, in general, and especially for the case of this thread, he will make a much better husband and father at 35 than 19, 25, or 30.
For her at 18, she will need help, from her husband, yes, but also, as I outlined, from the rest of the older family members.
So, date her when she is 15-16 or so and marry her when she's 18.
I'm not nearly the first person to observe or even recommend such age differences.
I know; I know; the currently strongly politically correct view is that we should ignore any such gender/age differences as I outlined. But, bluntly, that's one of the reasons we're rapidly going extinct, literally, from birth rates too low from bad family formation.
It's just the facts of life.
I once believed as you did: At 18 she was fully ready to be married and a mother, totally awash in love. Would have made a terrific mother. But I was only 4 years older, and we planned. Lots of good, rational planning. She was brilliant, Valedictorian, Summa Cum Laude, PBK, Woodrow Wilson Fellow, NSF Fellow, mathematical social science Ph.D. from two of the world's top experts. But that's NOT what Mother Nature wanted. It flopped. She died. Her effort to be equal ("this equality stuff got me in a lot of trouble") was fatal. Now I'm lonely. Never, never, never again, never should any man ever, ever, ever consider a marriage with two Ph.D. degrees and close in age. NEVER. It's begging for disaster. In a word, are trying to do without the "zing".
Early on she said "Women don't have to be just cared for. Women can do things, too." Well, that was a deeply convoluted statement: Being cared for is what she really expected, wanted, and knew she needed. But she wanted to pretend to me that she could also paddle our little canoe. She knew she wanted to be cared about, cared for, taken care of. And for young women with children, those feelings are nearly universal and universally strong. So, you just learned one of the facts of life.
Ages 35 and 15 have a LOT going for them: Then at 38 and 18 they can be married and right away start their family. GOOD. TOUGH to do better than that. But you look at this terrific situation and find something wrong -- you need to study Women 101 for Dummies -- Men.