Life expectancy is a statistical measurement. There are typically two forms of life expectancy used when discussing it which is life expectancy at birth and the life expectancy of someone who has reached a certain age. These measurements are usually very different.

A general way to understand it is by taking a population of say a hundred people. Let's say that 60% of them will die before the age of 15 from disease, war, famine, or some other cause. So you're left with 40 people after 15 years. Those 40 people can expect to then live for another 40 years to the age of 55. Notice the two separate life expectancy numbers.

When all 100 people in your original population have died you'll notice that in spite of the fact that 40 people lived to be greater than 30, the majority of people died at some point before the age of 15 which skews the average age quite significantly to somewhere around 30. This is the life expectancy at birth.

Being told "the life expectancy at birth was 30" does not tell you anything about the soft age limit of the humans of that time period. It just tells you that an average human being would die before they reached the age of 30.

So, being a statistical measurement, life expectancy takes into account various demographic factors and has no bearing on the generally soft human age limit. Asking about life expectancy for a transplanted person is then very difficult because of specific biological issues and the general fact that statistical measurements have very little bearing on individuals.

That being said, to answer the heart of the question: human beings in the ancient world had generally the same soft age limit that we do today. The increase in the statistical life expectancy from the ancient to modern world is due to advancements in various fields like medicine, general hygiene, and diet.