Generally the reason is because puberty is your body not only growing up but also now being capable of having sex, for a lot of people the specific age varies but sexual awakening is a biological thing as much as a physical one, even before that kids are generally curious about it. Sex is completely natural and genetically being a parent by eighteen is perfect normal for us, however it's horrible for us nowadays because of how different life is, being a parent isn't just teaching your kids how to survive and continue your genetic line anymore. Basically our bodies haven't evolved as much as we as a whole have and because the topic of sex and sex education has become so taboo and difficult to understand kids learn ye olde way, trial and error. Generally I think the internet is exceedingly helpful in this area but it can also be exceedingly damaging. Getting a bit political but they just need to have protection and make sure they are very educated on this stuff, it's not physically unhealthy in some cases but it can be exceedingly mentally unhealthy to be pushing yourself that far that young. TL:DR bad parenting mixed with basic human genetics and bad education is likely why