Two things are necessary for a private pilot to act as Pilot in Command (PIC) of an aircraft even if they are flying alone; a current flight review and a valid medical qualification.
To fly with or without passengers, your most recent flight review must have occurred within the past 24 calendar months. This means that whatever day of the month you completed your last flight review, it will expire at the end of the same month 24 months later. After this date you may not act as PIC.
There are a few exceptions to the requirement for a flight review. If you take a pilot proficiency check or a practical test for an additional certificate or rating with an examiner you will have 24 calendar months from the date of the exam until you need another flight review. Also, a completed phase of an FAA-sponsored pilot proficiency program, such as WINGS, satisfies the requirement for a flight review.
You also cannot act as PIC without a valid medical qualification. There are several options that meet this requirement. You can get a first, second or third class FAA medical certificate from an Aeromedical Examiner (AME), or you can follow the requirements for BasicMed (14CFR Part 68). There is an exception to this rule for Sport Pilots who can fly light sport airplanes with a current and valid U.S. driver’s license.
For the full details of the Flight Review regulation, see 14 CFR 61.56 at https://www.law.cornell.edu/cfr/text/14/61.56.
You can find more information about FAA BasicMed at https://www.faa.gov/licenses_certificates/airmen_certification/basic_med/
Beth Rehm, CFI
JB Aviation Flight Training