The common way that we measure the thickness of hair is based on measurements of the individual hair fibers diameter. The picture below illustrates what we measure on hair using specific equipment (imagine a line passing through the center of a hair fiber).
To have a good statistic on the changes of hair diameter (or hair thickness) with age, many hair fibers are measured considering different ages, ethnicity and gender. Based on all that data, we know that baby hair is the finest, children’s hair is coarser than baby hair and adult hair is even coarser. With advancing age, hair fibers become finer and in general don’t grow as coarse as in prior adulthood.
Scientific data shows that, in average, the hair thickness increases until about 43–46, hitting its peak around that age, and then starts to decrease with increasing age (What women want – quantifying the perception of hair amount: an analysis of hair diameter and density changes with age in Caucasian women). The change in the diameter of each hair is small (and varies from fiber to fiber), but still it is enough to be perceived as a lower hair volume with increasing age.