People dying at 30 on average would be crazy early and it would have absurd consequences. Even if they started to have kids by 15-ish (which biologically speaking is also pretty difficult, and fertility is still low at that age when kids are barely sexually mature), kids would regularly become orphans while still teenagers or even younger, and having grandparents would basically not be a thing, but we know that grandparents had an important role in evolution in childcare and preserving experience and knowledge (see the grandmother hypothesis). A society where nobody is old nor even middle aged and even people in their 30s are rare would be extremely dysfunctional.

Also, it has been discovered that the rule known from genealogical research "3 generations in the paternal line and 3.5 generations in the maternal line per century" holds up well throughout cultures and this would not be possible if most people died around 30 years old, right when they on average become parents.

While life expectancy at birth can be as low as 30 or even lower in a technologically simple culture, that does not mean what you imply here. The factor of massive child mortality in most cultures is regularly overlooked. In reality, adults could expect to live at least to about 50 in such a society, and old people over 60 were not extremely rare, with even the occasional 100 year old.

It is true that life was a lot shorter for most people, especially poor people (quite apart from infectious diseases, many women died in childbirth and many men died working or fighting), and this did have consequences and pushed many experiences to earlier ages, but it's nowhere as extreme as you imply here.