"However those who survived to adulthood had a life expectancy considerably closer to modern ones than most people think."
Nope. I see this consistently repeated all over HN or Reddit, but it just isn't true. Only the high clergy routinely reached their late 60s, and trying to analyze general mortality from the relatively sparse ancient records that mostly concerned the richest and most important people introduces enormous biases.
It is the burial grounds that tell the full story without major biases (there still are biases caused by, say, sailors drowning and their bones not being buried on land - a nontrivial cause of death in a maritime imperium). And the bones of average people indicate that over half didn't live to see their 60th birthday until the 19th century.
In some really bad periods (the Dark Ages in Europe), the average buried adult person is in their early forties - a tale of food insecurity and constant raids. Rome, at least, was mostly food secure and had long periods of peace, so celebrating your 50th birthday was very thinkable for a free Roman citizen of non-pauper status.
Edit: someone downvoting this. People, just take any paper about an archeological excavation of an old burial ground and the detected age distribution of the skeletons found. People over 60 are untypical, though always present in all periods until well into the industrial revolution. And early Medieval cemeteries skew a lot younger than either Rome at its height, or the Late Middle Ages with their better agriculture and more capable governments.