The woman's age from 18 to 45 (Chinese reckoning) is on the top line
while the months January through December indicate the month when the baby is conceived.

By following the chart you will be able to tell, in advance,
whether your baby will be a boy or a girl.

This chart has been taken from a Royal tomb near Peking, China.
The original copy is kept in the Institute of Science of Peking.

The accuracy of the chart has been proved by thousands of People
and is believed to be 99 percent accurate.

For example: If you were 40 years of age when you conceived in February
according to the Chinese Lunar Calendar, your child will be a boy.

The chart is based on the month the baby is conceived and not on the birth of the baby,
B-Male, G-Female.

To arrive at your Chinese Lunar Age:
I believe the way to calculate it is this:
If you were born from January 1st
- February 20th (inclusive of these dates),
you should use your real (Gregorian/English Calendar) age.
However, if you were born from
February 21st - December 31st (again, inclusive of these dates),
you should add one year to your Gregorian age.
For example, if Linda was born January 12th, 1976,
her Chinese Lunar Age would be 27.
But, if Alice was born on July 7th of the same year, 1976,
her Chinese Lunar Age would be 28.
I hope I didn't make this too confusing!
I do not claim to be an expert, but this is the method that most Chinese Lunar sites that I have come across suggest. :)
J