I’ve read that the average life expectancy for ancient Romans was 35 years. If Roman soldiers were held to serve the army for 25 years, then for most soldiers that meant until they died.
That 35-year figure was probably life expectancy at birth. Infant and child mortality rates were a lot higher in those days, so if a boy survived to be old enough to become a soldier, he’d have a pretty good chance to live well beyond 35.