Here is why

Let n be the number you picked, and Y be the year you were born, so the operation from 1 to 6 can be written as:
(n*2+5)*50+1763-Y , which can be simplified as 100n+2013-Y, which is the result: with n as the leading digit, and 2013-Y be your age.
As long as you are less than 100-year old, the formula will give you the right answer.