While I think the 1/2 + 7 principle is practical, I wouldn’t use it as a rule.
There’s nothing wrong with a couple of 18 year olds getting married as long as they are mature, frugal, and willing to muck their way through. I know a lot of people in my community that got married right out of high school and do alright. Granted, many of them are farmers and the husband has been working for their family since they could walk basically.
I know homeschool families around here where their teenage sons will do their studies in the evening and hold down a full time job during the day. One of them I know is 16 and is already graduated with a healthy little nest egg he’s been building for a couple years.
Both my husband and I came from blue collar backgrounds. Both of our parents started off dirt poor. My husband has always been blue-collar. He doesn’t get paid a lot compared to most bread winners. He figured he would never get married because of this. Most women are vain and expect a certain amount of pampering as the norm. By doing so they overlook some really good Catholic men.
We have 3 daughters and 1 on the way. Our goal is to raise them in a way that they are frugal and useful to their husbands with every skill they will need to hold down a home on a limited budget and to do it with great love and joy. Cooking, sewing, gardening, preserving, budgeting, etc. I love and greatly admire my husband who works with his hands and who can build and fix most anything. He doesn’t make a lot of money but we also don’t need as much money because we do things ourselves. We don’t want to limit our daughters to the big earners, who, to be rather frank can often be quite dull in personality and lacking in character and admirable qualities. Our hopes are that they would be suitable partners to the humble farm boy who works hard by the sweat of his brow and is earnest in his faith and duties.