Depends on how many 5-12 y/os I package each year and their age distribution. Harvests vary from year to year. Averages are taken over a wide enough window to smooth out the bumps. Hard winters, drought, disease can also take a toll periodically. In general any particular ecosystem can only support some stable population. Dieoffs tend to be the rule that maintains it. The young and old are the most probable to die off. The average age is essentially constant.
If a population has always contained 1,000 individuals and, over a thousand-year period there have been 500,000 births and 500,000 deaths, then (ignoring phasing effects) the average lifespan of the population will be two years. Even if 999 individuals live 1,000 years and the other 499,001 individuals live for 63 seconds each, the average lifespan would still be two years.
Likewise with lifetime birth rates. If there are 1,000 individuals and, over the course of a decade, one of them has 5,000 offspring and the others have zero, the average birth rate would be five per decade.
These averages are completely unaffected by distributions. It may well be that they do not represent typical members of the species, but numerically they are what they are.