It's a common misconception that people in earlier times lived much shorter lives than we do now. Historically, the biggest single factor influencing life-expectancy at birth was infant mortality. Hence, if you survived from childhood into adulthood, you could normally expect to live well into your 70's (barring major episodes of disease outbeak, warfare, or natural disasters).

Since prehistoric times, the length of a typical human lifespan has remained fairly constant. Nowadays, recent advances in modern medicine are helping to extend peoples' lifespans into their 80's and 90's - i.e. beyond what was previously normal. However, cross-culturally there is probably a lot to be said for patterns of integration and cohabitation between older and younger generations, in terms of how well newborns and elderly members of a community are cared for.