A lunar year is defined as a certain number of phases of the moon. To calculate your age in lunar years, you'll need to know the time between lunar phases, called the "synodic month," which is approximately 29.530 Earth days. Twelve is the usual number of phases per lunar yearthe Islamic calendar being the prime example. The Chinese calendar also uses lunar months but its new year stays pinned between Jan. 10 and Feb. 19instead of roaming through the entire Gregorian year, as the Islamic calendar does. In other words, the number of phases varies between Chinese years.
Islamic Lunar Years

Multiply the number of years you've been alive by 365 days.

Add an extra day for each year divisible by four (leap years).

Add up the days since your last birthday and add that to the result in Step 2.

Divide the total in Step 3 by 29.53.

Truncate the decimal places. That is your age in lunar years.
For example, if the result of Step 4 is 43.23, you are 43 lunar years old.
Chinese Lunar Years

Look up the date of the new year in the year that you were born and of the current year.

Subtract 1 from your birth year if you were born before that year's Chinese New Year's Day, as determined in Step 1. For example, if Chinese New Year was on Jan. 31, 1976, and you were born on Jan. 16, 1976, then use 1975 as the basis for determining your age.

Subtract 1 from the current year if the Chinese New Year hasn't occurred yet for the current Gregorian year.

Find the difference between the results of Step 2 and Step 3, and then add 1. You add 1 because the Chinese count the time in the womb toward your age.
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 Photo Credit earth and moon image by smn from Fotolia.com