For mRNA vaccines (Pfizer and Moderna), the FDA and ACIP both recommend boosting for all people 65 years of age and older, for people from 18-64 with comorbidities for severe COVID-19, and for those whose occupational exposure puts them at high risk of COVID-19.
For the J&J vaccine, they recommend a second dose of vaccine for all people over age 18 who received the initial dose. People who had any complications from the initial J&J vaccination such as venous thrombosis should get a second dose of an mRNA vaccine rather than a second dose of the J&J vaccine.