She won't get any marriage proposals while you are playing the current generation. At 18, a child automatically gets sent away to college (you'll get a notification) and won't return to the house. Once all children are gone, the parents will toddle away in the house doing what they do until you see one or even both of them lie down on the bed and fade away, indicating they have passed on.
At that point, you will get the family tree with an icon to start the next generation. Click that and you will get the first child to look at. Go through each child (now an adult) and see which one you like best. It will list their age, occupation, salary, current savings (which is exactly the amount the now-deceased parents had when they died), their likes/dislikes, and whether or not they want children. You can keep cycling through them again and again, but once you hit Adopt, that is the one you get.
The child-now-adult moves into the house, and the marriage proposal usually comes within the first 5-10 minutes of play, often within the first 5. I've rarely had to wait very long. But if you reject the first proposal, you may have a longer wait until the next one.
If you find your little people are having trouble getting their next child (it's harder as they age), you can always adopt if you have the funds. I like to have at least 3 children so that I have choices when it's time for the next generation, but that's just me. Sometimes an orphan might show up on your doorstep; it's kinda rare but it happens.