At a minimum, a private pilot must have a current third-class or higher medical certificate when acting as pilot in command.
A current first-class or second-class medical certificate is an alternative, as these two qualifications are more stringent. However, a first and second-class medical certificate are generally reserved for commercial operations. If under 40 years of age, the third-class medical certificate expires after that 60th month. If 40 years old or older, a third-class medical certificate expires after the 24th month of the date of issue.
14 CFR 61.23 Medical certificate: Requirements and duration