That's incorrect. The average age of adult death was much higher, around 60 [1], ie. if you lived to 5 years old, you could expect to live another 55 years.
That was in the mid-19th century, which was a very different world from the Medieval period. From your own link, during the 18th century Swedish 10-year-olds lived on average into their mid-50’s. From that it can be reasonably extrapolated that 10-year-olds lived on average into their 40’s in (civilizations at the level of development of) Medieval and Classical-period Europe.