If the worry is "not enough young people", what is the logical continuation? When we produce lots of young people now, and those become old, we need even more young people, and so on? So population growth is the only way forward?
Not even mentioning that today fewer people are needed to take care of other people than in former times. For example one farmer can produce food for many, many more people than one hundred years ago.
It's obvious that population can not grow indefinitely. Just think of the surface of the earth. There is a limited number of square meters. Eventually you would have so many people that there would be one person per square meter.
Maybe we could even live like that. We could live in skyscrapers, or even in little boxes Matrix style and spend our days in spacious virtual reality.
But the question is: do we want to live like that? We have to also think how we want to live, and how that can be made possible.
In the same vein, I guess the world obviously can not be overpopulated. If if were overpopulated, people would start dying until it would not be overpopulated anymore. That's presumably what is already happening in some regions of the world anyway, and has always happened. In "Collapse" Jared Diamonds describes a tribe living on a small isolated island. When they get overpopulated, some members simply had to get into a canoe and paddle away, to die on the sea alone. It's not a new problem.
But again the question is, how do we want this to play out. We can let people be born and then starve to death, or we can not let them be born in the first place.