Why is it that girls and boys can reach puberty at such a young age?

It's not like we are going to be reproducing as young as 8 for a girl or however old for a boy.

It's not like we are going to be reproducing as young as 8 for a girl or however old for a boy.

According to the science, puberty is premature due to the lifestyle and diet changes over the years.

In particular, weight gain due to fatty and sugary diets sets off puberty early. This is also something that apparently speeds up aging (a double whammy).

If they were on a healthy diet, you should see them reaching puberty at 12-13 years old, as you would expect.

Puberty isn't something people can consciously control or decide to happen. It's something that should naturally happen, but it can also happen due to unnatural causes.
Then following on from what you are implying; just because you have reached puberty, it doesn't mean that you should have kids. There was a thing about rising teenage pregnancies a few decades back, so the government implemented laws to only permit certain activities after minimum school leaving age. Reaching puberty allows you the ability to have kids, but having them would depend on the individual. What makes a person ready to have kids will depend on a long list of factors, many of which will require lengthy discussions on laws, philosophies, biology, and psychology (essentially a can of worms).

I think the more appropriate question is whether something should be done about it, and if so how it should be dealt with.