You can’t put rules to it. I’m marrying the first girl I ever dated for more than like 2 months. She was 18 when I met her, I was 19. I’m going to be 25 when we get married. There’s too many variables, like each individual, their past (which tends to heavily influence how people view romance), their current situations, etc.
However, good luck cultivating a healthy enough relationship to move towards marriage if other parts of your life are fucked up. (Money, about to enlist in the military when you meet, got a baby momma, you’re the type of person to wait in line for days for a new video game, etc)