I've got a 19" CRT and am over 50 so I'm not sure that's it.
Use visual references (i.e. the little waves in the water, trees, shrubs, your shadow, anything) to give you a sense of scale. I was having the same problem when the terrain first got updated because all the visual references went away in that flat green terrain. Where there are no references, or as dusk falls over water and the waves become obscured it's harder and you have to be more careful.
Go out over water, or sea level terrain and set auto-pilot for the lowest alt your comfortable flying at. Get a sense of how big those visual references are and how close you can come to them. Remember, most trees are ~30' tall so you can easily fly below treetop level although I usually try to avoid it (I've bent my prop on both trees and buildings before without other damage and had to ditch). Set this image in your mind and you're good to go.