The Date objects now() method parses a date string and returns the number of milliseconds between the date string and midnight of January 1, 1970.
var millisecondsBetweenNowAnd1970 = Date.now();
For example: In the example below, we pass a date string, which lets assume is your date of birth to the parse() method, and the method will return milliseconds between my DOB and midnight of January 1, 1970.
Then using the Date.now() method you can get milliseconds between present date-time and midnight of January 1, 1970.
Using this you can get, a persons age in milliseconds, from there the conversion to age in years, months, etc becomes super easy.
Full tutorial & Video demo: Calculate age of a person using Date.now() and Date.parse() method.