Determining the demographic structure of turtles is important to understanding their population status and conservation needs. Concern has been raised for the long-term persistence of the Western Pond Turtle (Actinemys marmorata) based on demographic analyses using size while ignoring age. Here, we compare the size versus age structures, and examine growth curves, for turtle populations from four sites in the Klamath–Siskiyou ecoregion of northern California and southern Oregon. We show that age structure does not correspond to size structure for two populations. Also, the most abundant of these populations had relatively few small turtles, which suggests inability of previous researchers to locate small turtles. Growth rates and adult size differed among populations, with turtles from two sites in the Coast Range significantly smaller and slower growing than turtles from either a reservoir on the eastern lower slopes of the Coast Range or the Klamath Basin east of the Cascade Mountains. Neither air temperature nor elevation explained the differences in size and growth rates. We hypothesize that larger body size and faster growth rates for some populations of A. marmorata may be due to high local productivity. We show that use of size alone gave an erroneous interpretation of population structure. Finding a few small-sized turtles in populations may not represent a lack of recruitment but, instead, a need to determine the proportion of young turtles based on their actual ages.