Follow these steps:

1. Think of a number from 1 to 9
2. Multiply this number by 9
3. Calculate 10 times your age
4. From the value obtained in point 3 subtract the value obtained in point 2.

The result is a number of 2 or 3 digits. If it has 2 digits, add them up and get your age. If it has 3 digits, add the first number that has 2 digits with the second that has 1 digit and you also get your age!