STEM hobbies are free-choice activities through which participating individuals may develop sophisticated STEM knowledge and expertise. To date, research into STEM hobbies and hobbyists has examined hobby groups by subject area. Missing from this body of work is research that examines the development and participation in different types of hobbies by age, ethnicity, and gender of participants. This research, part of a larger series of investigations of American adult STEM hobbyists, aims to fill that gap. Surveys were completed by 2,838 respondents from ten different STEM hobbies (astronomy, beekeeping, birding, electronics/robotics, environmental monitoring, falconry, gardening/horticulture, home brewing, model building, rock/fossil collecting). Results showed that there is great variation between STEM hobby groups across multiple variables and divergent descriptive profiles emerged for each group. Results also showed that, in several groups, more than half of the adult hobbyists reported first participating in their hobby during their youth. This study illustrates how crucial childhood experiences are to encouraging lifelong explorations in STEM and provides information about the types of experiences hobbyists reported as being influential to their hobby-related learning and development. This information may be used by educators and organizations to design programs to support current and future hobbyists.