Who needs a Third Class certificate
You need at least a Third Class medical certificate to exercise private or recreational pilot privileges, to act as pilot in command or required crewmember as a flight instructor in operations other than glider or balloon, and to exercise student pilot privileges.
The standards are the most accommodating of the three classes, and the certificate lasts the longest. For most private pilots this is a short, routine appointment.
Third Class duration
Measured from the last day of the month your exam took place.
Under 40 at the exam
60 months, or five years
Age 40 or older at the exam
24 months, or two years
Age at exam is what counts
Turning 40 during the validity period does not shorten a certificate issued while you were 39.
Source: 14 CFR 61.23 (eCFR)