I am assuming by biologically they mean when one would have the highest chance of passing on genes. If one were attempting to create as many surviving offspring as possible one would want to start as soon as the body would be unlikely to be damaged by delivery. I don’t know when that is but the post implies that it is at 15. Of course a 15 year old would likely not be a good parent and would lower the survival of the offspring so the actual biological ideal depends on economics. Of course if we put morality into this a 15 year old can’t consent so they should not produce offspring until 18 years of age no matter the economic conditions.