If you lived to adulthood at all back then you would likely make 70, not just if you made it to 30. That low life expectancy was due to extremely high infant and childhood mortality rates, which modern sanitation almost singlehandedly ended. Some places, they didn't even name infants unless they made it to childhood.

There wasn't some great culling of weaklings in their 20s. Also, social programs don't deserve the credit you give them, because extended families and communities used to look out for their own.