If nature wants us dead by 30 or 40 there's no one to look after offspring unless you get your kids when you're 15
Or you get kids with a 15 year old girl when you're 45ish

Well people actually bred very early back in the day and that short life span was enough to make sure that just fittest offspring survived. They didn't care about finishing a college, getting a job and being a "proper citizen and human being" lol... it was more important whether you can kill well and not get killed.
It's our times phenomenon - living way over 30.