Why can't most organisms reproduce until they are fully grown?

Answer 1

Because, in evolutionary terms, those that had offspring after they were fully mature, were more successful at it. They were able to better protect their offspring, and in doing so, they protected their genetic material, ensuring continuity.
As an example - assuming that, in days gone by, people had children as soon as they were able to, then those who matured later would naturally have children later, and would be better able to protect them than someone who had offspring at a younger age. This would produce a selective pressure to mature after growth had occurred.
Similar pressures would apply to many higher organisms.

Answer 2

Obviously the state of "fully grown" takes a very different amount of time for different organisms.

So a cat is "fully grown" and able to reproduce within a year, while humans are not typically physically capable of reproducing until their teens. And bacteria can divide once every 10 minutes.

It is a matter of becoming physically mature enough to undertake reproduction. Cats can reach sufficient size quite rapidly to be able to reproduce, while humans take a lot longer.
Additionally, in highly-social animals, where an important aspect of maturation is *learning*, reproduction will depend on being able to learn enough social skills to mate successfully, and to be able to bring up your child successfully. For example, it is not unusual for apes to mate successfully for the first time, but to not know what to do with their child, and so to accidentally kill their first offspring through neglect. Subsequent offspring are more likely to survive.

Answer 3

Probably because in your head you're just defining 'fully grown' as the time when an organism reproduces.

14 year old girls and younger have children - do you really think they're fully grown?

Answer 4

They don't produce until fully grown because they have other biological tasks they must do first. There is a natural order to things. It is important to finish the tasks that lead to survival first and then procreation is accomplished

Answer 5

Because they need to concentrate on growing. Reproduction, with all its courtship, gestation and offspring raising is very hard work.