Final answer:
A third-class medical certificate for individuals under 40 is typically valid for 60 months, but this can vary depending on regulations and the specific country.
Step-by-step explanation:
If you are under 40 years of age, your third-class medical certificate is valid for a period that varies by specific age ranges. For those between 20 and 30 years of age, the certificate is typically valid for 60 months (5 years). However, it is essential to check with the appropriate aviation authority, as regulations can change and may vary by country. The most basic medical certificate is Third-Class Medical Certificate and is required for pilots who operate aircraft for personal or recreational purposes.