How do you counsel younger patients regarding long-term radiologic monitoring of non-functional adrenal adenomas?  

I do not think biology is so black and white. The risk of malignancy in adrenal masses according to size is a spectrum and certainly not zero just because at one point in time it is less than 5 cm (how about 4.5 cm?). Unless the radiologist can confidently diagnose a myelolipoma I think clinica...

Benign non-secreting adrenal adenomas (< 20 HU on non-contrast CT) < 5 cm in size don't need follow-up imaging. Surgery is recommended for adenomas > 5 cm and > 20 HU on CT to rule out malignancy.