I'm 52. How many years of experience should I put on my résumé? Should you not go back more than 15 to 20 years to prevent ageism?

Your résumé should have bullet point support for positions going back 15 years, give or take few, depending on where a good break is in your job chronology. However, you don't want to come across with less-than-desired experience. Therefore, add a final job saying "Prior experiences include gaining strong skills in ________, ________, ___________, and ___________. No need to list companies or jobs. This way, you avoid ageism yet boast about additional skills you bring to the future company.