Yes, so few forests are old in proximity to tracks, past clearing, disturbance and fire are all issues.
For my layout, I'm targeting 3.5-4" as the tallest trees, so about 40'. This is generous in my prototype and avoids the trees overwhelming the backdrop. When 9000' mountains are 14" on the backdrop, the trees can't be 7".
BTW, does anyone know of a good color chart for the Scenic Express turfs? The online colors are not very helpful. Looking for a realistic conifer color and their conifer green is too dark.