Infections yes. Dysentery was mostly a disease of the military and cholera was at the time limited to the Ganges plain, did not exist in Roms.
I can certainly see that quote as being a realistic one.
Life expectancy in England around 1910 was appalling, with huge infant mortality - getting on for 60% did not make it to their teens.
Remember that this is in a country that was supposed to rule much of the world and was a genuine superpower, yet we could not keep our children alive - just 4 years before WW1.Things didn’t really improve until the very late 1930’s
Life expectancy in England during the 19thC was so low that the main form of life insurance to the lower orders - via friendly societies - would not take on new clients above 40 years old.
I have quotes from various works by various Victorian reformers such as Dr Robert Baker that put the average lifespan for workers in Northern England towns such as Bradford, Leeds and Manchester during the 1830’s at 19 for unskilled labouring families, at around 27 for skilled working families and 44 for those in better occupations such as professional classes and gentry. The link between occupation, classes and life expectancy was extremely clear.(in fact Bradford was the very worst in the country, you might be advised to try search this out as I did some years ago)
Take a search for some of these Royal commission reports
1845 ‘Report of Commissioners of Enquiry into the State of Large Towns and Populous
Districts’, James Smith
Edwin Chadwick on “The Sanitary Condition of the Labouring Classes”
(1842)
Also take a look at this .pdf
HIDDEN BENEATH OUR FEET: The Story of Sewerage in Leeds
Put in a search for ‘UK Mortality rates for the 20th Century in England and wales’ - that will get you a link to a .pdf file - download and read it, then look at table 2 and examine the numbers for year 1901, look at the mortality rates for children up age 15.
When you have done that, then consider this, that figure is a combined total for all UK children, including those from better backgrounds - and then you can imagine that those industrial towns, where conditions were much worse than average would have had mortality rates far higher than the national average
There is plenty of evidence of low life expectancy around, you just need to look for it before decrying the OPs quote.
I don’t know about the veracity of the OP’s book, but between high infant mortality rates and lead poisoning, I could see this being the case.
Minus lead, as I understand it, the average life span of humans isn’t all that much different for a person in modern day versus someone 1000 years ago, once they make it past infancy. But if you’re drinking lead, eating off of lead, and etc., I could see that bringing everything down the post-infancy average.
I sometimes ask people to look at Psalm 90:10 in the Bible: “Three score and ten years are a man’s days, or eighty if he is strong.” Less poetically, King David was saying, “An average man may live 70 years, maybe 80 if he’s lucky.”
In other words, even in ancient times, people who managed to survive childhood illnesses could usually expect to live about as long as our grandparents did.
Or, more poetically:
You get your chance to try in the twinkling of an eye: Eighty years with luck or even less.
And a cite:
Working my way through the book “Jerusalem” by Simon Sebag Montefiore. While many of the casualty figures may have been exaggerated, the number and frequency of deliberate deaths is pretty high for an extended period through history. Romans, like many other conquering nations, when they weren’t attending to sewers or viniculture, would routinely slaughter and perhaps enslave inhabitants of any town that resisted a siege. That was par for most wars over the last few thousand years.
Considering the level of internecine murder you can read about inside the Roman imperial families, war was not the only place that death happened. The only exception might be that lower class families who indulged in murder might have to answer to the magistrate.
Yes, they sometimes were.
A large number of the Roman soldiers were on garrison duty – basically guarding the already conquered territory, acting as bodyguards for the province governor, keeping general order (serving as the local police/national guard at times), and generally showing the flag for Rome. Except when they had to deal with a local rebellion, or in real emergencies, when they might be transferred to somewhere that had current fighting going on, often these garrison troops were reasonably safe.
Even troops stationed on the borders of the Roman Empire were often not in great danger. Most of the borders were reasonably settled, and Roman troops were there as a show of force, to keep the ‘barbarians’ from starting anything. Which only happened occasionally.
This was especially true during ‘peaceful’ times in the Empire. Other times, it was the romans who decided that they wanted to conquer some other territory, so they started the aggression and crossed the border. And of course, later on, when the Empire was crumbling, it seemed like barbarians were attacking from all directions.
It really depends on where you were and the era. Serving on the Euphrates frontier meant quite a lot of action from the early second century onwards.
Serving on the Rhine-Danube frontier meant periods of war and peace.
Serving at Hadrians Wall, or Limes Arabicus was pretty much a cushy posting for centuries.
One of the main killers, as always, was malaria. Some people estimate that about half of all people who ever lived died from it. ‘roman fever’ is thought to have been a particularly deadly strain. Historians have suggested it as a major factor in the fall of Rome.
Re: Legionary survival rates:
Don’t forget civil wars. A lot of Rome’s wars were against itself. Even when the frontiers were quiet, you might find yourself squaring off against another Roman army.
Also, disease would be more likely to kill you than enemy action.
A figure I see thrown around a lot is 2/3 chance of surviving your service time. That’s not a great cite, though, sorry. 
Which is why the ancient Romans drained the Pontine Marshes near Rome…after the fall of Rome, the drainage system was neglected-it was not drained again until Mussolini did it in the 1920s.
There were some Romans who survived into extreme old age-are there any Romans who made it past 100?
Should have made it clear that I was pointing to the legion’s enlistment commitment as being against reading the stat and thinking most people died at age 20, so my bad.
Found the closest answer to this in A Companion to the Roman Army, which states;