1) Ask the  person to reverse the last four digits of  their phone number to produce a new 4-digit number. Subtract this new number from the phone number, or vice versa, depending on what number is smallest. Don't ask for the result!

For example, a phone number such as  *** 4567 would become 7654, so subtract 4567 (the smaller phone number) from 7654 (the larger new number), leaving a result of 3087.

2) Ask the person to add together the digits of the resulting number and to repeat the procedure until only a single digit number remains. Don't ask for the result!

In our example, 3267 would become 3+0+8+7 = 18, and 18 would become 1+8 = 9 (a single-digit number).

3) Finally, ask the person to subtract this single digit  number from the last two digits of  their year of birth.  Ask for the result!

You will then be able to tell them their age, and the year in which they were born!

How does it work?
The final number remaining from the calculations in 1-3 is always 9. Therefore secretly add nine to the result they give, and you will be able to tell them their age!   Try it out on someone who doesn't really want to reveal their age to you - such as your teacher.