I believe that an age limit on giving birth would be justified. Women over 40 have no business having babies at that age. It is selfish on their part. If a woman was 50 when she had a child then she would be 60 when the kid is 10, the age of his peers' grandmothers! Older women don't have the energy to be taking care of babies.
When teenagers have children they become more likely to not finish high school and to need welfare. This means a portion of taxes must now go to that person for their irresponsible action. The children of underage parents have an increased chance of also needing welfare when they grow up. They also have a higher probability of committing crime and being incarcerated. These negative impacts affect the lives of every tax paying member of a society and are an unnecessary burden.
To me, though it sounds like it could be good in theory, sounds a little too harsh and constrictive. I think the better thing to do is have contraceptives widely available and for free, and it would cut down drastically on people who are too young giving birth at an inappropriate age.
This question is a bit absurd, because conception is not something that can absolutely be controlled, unless you were to sterilize people beyond a certain age. But the question is about whether the limit would be just, not whether it is possible. To my mind, it would not be just. Childbirth is ideally a decision best left to the woman giving birth. Society has many interests that could be seen as good reasons for limiting childbirth at either end of the age scale because of largely financial concerns. A young mother perhaps cannot afford a child, an older mother may have complications. Still, I think that this is a matter of personal choice, and for society to interfere would unjustly interfere with individual freedom.
No, I do not believe that an age limit on giving birth would be justified. One reason I believe this that it would be utterly impossible for the government to control whom is able to give birth. If it could be controlled there would be less teen births today. Furthermore, how would you accomplish this feat?