20 years old is a mere baby with little experience in life when you come at it from my rather older perspective. Of course no 20 year old thinks that is the case, I didn't myself when I was that age...
Except for the fact that physically you've almost stopped growing at 20, and however much you could deny the fact, there is a point when you stop picking up new skills as quickly as you did when you were 20. My brother is 10 years older than me, he did similarly well to me in high school and so on, and I remember as a young kid watching him picking up new things really quickly - he taught himself how to play the guitar and he is really quite good. On the other hand, nowadays I pick up and learn stuff a lot quicker than he does.
Of course, with experience comes wisdom. But you can't seriously be proposing that if me and my father both decide to teach ourselves how to speak Dutch, that we will learn it at the same rate. That if we compare our performances in sport that he gains muscle as quickly as I do.
(Ironically, yes, I am almost 20 myself
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Common misconception.
People did get to 60-70-80 years or more. The low averages come from epidemics and diseases in general, but especially a very, VERY high number of stillborn babies or child deaths under 2 years old.
That, and fantasy games usually have healing magic, which would work wonders for all the things that would cause people to die early.
A common misconception? It is a statistical fact that the majority of the population died sometime in their 30s. Diseases, predation and so on are important factors which affect numbers of a population. You can't discount them, because they are what governs the mortality rate, and hence the average age and umbers in the population.
Speaking from a biological standpoint, nature doesn't WANT more members of the population to be post-reproductive age. Why? Because they are taking the resources that the younger members of the population need in order to survive and reproduce themselves. In this age, it's fine to have 60% of the population post-reproduction, because as you said, we don't have the same diseases, threats of predation, threats of starvation as our ancestors did. What happens to those countries with such a statistic? The population rapidly dwindles because the mortality rate is higher than the natality rate (number of deaths is higher than number of births)
The low averages did not come from high numbers of stillborn deaths - they came about because in order for a population to survive, the number of pre-reproductive individuals has to be very big so that some of them can die before they have grown into reproducing individuals and it won't affect the survival chances of the species. Likewise, the number of post-reproductive individuals is necessarily small because otherwise they would use too much food and so on and the species will die out.
In TES, they aren't going to adhere to a modern model of population, they'll adhere to a model from a comparable time in earth's history.