To calculate the age in years, months, and days based on a birthdate, use the DATEDIF function with the 'y' (for years), 'm' (for months), and 'd' (for days) unit types. The formula syntax is: =DATEDIF(birthdate, today_date, 'y')&' years, '&DATEDIF(birthdate, today_date, 'ym')&' months, and '&DATEDIF(birthdate, today_date, 'md')&' days'.