In salmonids, age at maturity is fairly genetically controlled. For example, WDG spawned 2-salt summers with 2-salts in order to get more big fish, as the more common was 1-salt, I think. In any case, the big fish were 1 year older. The result was that the got the bigger fish back but fewer because of another year's worth of mortality.

A 7 year old Chinook is gonna be a whole lot bigger than a 3 because of 4 more years of growth. You will remember that these fish evolved in an ecosystem where the high seas harvests did not occur, there was more food.

We now see smaller fish at age, due to lack of food. Not as much to eat. We see fewer old adults because of fisheries and the loss of those old-fish genes as the legacy of past fisheries.