The answer to this question depends on many factors including the amount of sun exposure you have, your baseline skin color, and your family history (if you have family members with skin cancer). It also depends somewhat on the number of suspicious looking moles you have. My gut feeling is that you should probably have an annual skin exam starting now. 20 years of significant sun exposure would probably warrant a regular check up with a
dermatologist. This will just involve a dermatologist checking all of your sun exposed areas for abnormalities and getting a
biopsy of anything that looks suspicious. For others that may be reading this, regular skin exams should be done for anyone with a lot of sun exposure especially for those that work outside. I would say that most people that are at risk should start getting checked in their 20s or 30s. For those with a family history of skin cancer, exams should begin about this same time (or younger if there is significant sun exposure). For those that have had suspicious moles, exams should begin yearly after the first mole is found. The best type of physician to do this exam is a dermatologist. I suggest that you schedule an appointment this fall for your first check.