Elevated glucocorticoids can cause a decrease in size of the hippocampus (a portion of your temporal lobe responsible for memory). However elevated glucocorticoid levels have NOT been linked to bifrontal atrophy. If you have bifrontal atrophy it is more likely related to age than Cushing's Disease. In patient's with frontal lobe atrophy, dementia is very likely. As such it is important to screen for the cause of dementia such as B12/folate deficiency, low homocysteine levels, and thyroid dysfunction. If your B12 level is low, it is very important to give you a supplement. If you have memory deficits, you can also consider asking your doctor for Aricept and/or Namenda. These are medications that used to treat mild/moderate dementia. It will not improve your memory, but it prevent it from getting any worse very fast. Finally it is very important to have an MRI once a year to determine the progression of the atrophy.
WIth regard to Cusings and Bifrontal Atrophy here is an article from the Journal of Clinical Neuroscience to back up what I said: doi:10.1016/j.jocn.2008.12.026
In terms of dementia, this can happen at relatively young age. The amount dementia depends on the level of atrophy and what areas of the brain showing atrophy. Patients with fronto-temporal atrophy can demonstrate the type of symptoms that you have mentioned. It is not age alone that lead to atrophy. Atrophy can also occur because of environmental factors as well as because of genetic reasons.