your skin actually gets thinner as you age. Think about how the caliper works: to measure the thickness of a fold, it is actually measuring two layers of skin and the fat in between. Using your age, most calculators attempt to estimate how much of the measurement is from the skin itself, and how much from fat. Most of the thickness (around 90% i believe) is from the dermal layer. This layer is full of collagen and elastin. Both of these compounds break down as you age, which results in thinning of the dermal layer. So to play with some dummy numbers as an example, for a single skin fold measurement:
20 year old, 20mm measurement, skin is 6 mm thick, 2 layers of skin, gives --> 8mm of fat
50 year old, 20mm measurement, skin is 3 mm thick, 2 layers of skin, gives --> 14mm of fat --> higher bf % obviously
As a disclaimer:
1. I made these numbers up for an example, I don't know what real numbers might look like
2. Skin thickness depends on many factors including genetics and sun exposure, so all calculators use estimates based on age, they may or may not be very accurate.
hopefully this is what you were after