They also can have their i.d changed to appear older (to get a job - common amongst daughters from the country-side).

Cut them in half and count the rings (it's what I tell my students they'll have to do to know my true age).

My wife is actually 10 days older than what her passport says, (her Father registered the wrong day back when).

When she got her British Citizenship we tried to get that corrected, HUGE problem, indeed at first they refused her citizenship because she said a different DOB to the passport, (I assume they thought she was a counterfeit or something) we had to go back a month later, when we tried to explain, it involved getting an affidavit from her Father, the Chinese Passport office, and a sworn statement before they would even consider it. Suffice to say, she now has two birthdays, her real one and her passport one.

There are too many variables, like solar calendar vs lunar calendar, so if she goes by the lunar birth day, according to the solar one it will fall on different dates each year.

Also, in China, when you are born you are 1 year of age, not one day as in other countries.  And topping that, officially all birthdays are on New Lunar Day, that day you are one year older, not on your birthday.

Assume, for the sake of an example, that New Lunar Day is February 10.  A child born on February 8 is one year old, and two days later, on New Lunar Year Day, will be officially 2 years old.

Are you confused enough by now?.   

Yeah this is quite common.  All of the examples you gave, I've come across myself.  They're not lying.  

If there's grass on the field then play ball.

it is common in China from my Chinese friends.

you think birthdays are confusing, in the middle east some countries dont put anything but the year born, so if you immigrate to america by marriage , refugee, student visa, whatever , the government gives you a birthday of july 1, the first day of the government fiscal year because they dont know their birthday at all. i tried to get a job in saudi arabia but since im   50 yrs old , they told me i could only work to 55, which is 62 on the arabic calender, very confusing, how do you know how old anyone is , not sure

Guessing a woman's age is always difficult. Especially when they dress older than they really are. Many people back home thought the girls in highschool were old enough to drink just because of the way they dressed and how they wore their makeup. 

Here it could be the same way, but the girls in highschool are in uniform so you know they are younger than 18. 

Look at the ears. unlike the rest of body organs, ears never stop growing.

Just pinch them to see if they are ripe. If you shake one and listen and then you hear loud screams, she could be too ripe. Women unlike fruit do not get sweeter, juicier and lose acidity with age.

The length of never shaven pubic hair will tell you the truth...most of the time.

Aside from this it's all smoke and mirrors.

If they are wareing nappies, you know they are too young. If they are no longer having periods, you know they are too old. Chinese women generally look much younger than Caucasian women. Be careful. The local Japanese restaurant has a cute little Japanese waitress. She looks under age (15) I asked her how old she was? 31 I could not believe it. Franck3