Add all three of your ages up, add the digits of the product, multiply by two, add the thousands digit of the year you were born in from the product, and there you go.
Just because I feel like being picky:
1. When one adds up numbers, one reaches a sum - NOT a product. Therefore, the sum of the three ages is 51.
2. If you add the digits of the SUM, you get 6. Two times six is 12.
3. I'm not sure I can even comprehend this instruction. How do you add something from something? Maybe you mean add to? Then we get 13...
.... just a note: I doubt a thirteen year old would have written these instructions.