Here's an easier version of the trick that's too obvious and lots of people would see right through it:
Take the first digit of your age, multiply it by ten, then add one, then add the second digit of your age and tell me the result. The result is 25? Aha! I know that your age is really 24.
To make it a little more sneaky, they added six instead of one, and they broke up the multiplying by ten into two steps, first asking you to multiply by five and then later multiply by two. And the icing on the cake is that instead of asking you to add six at the end, they had you add three just before you multiply by two, which has the same effect.
If AB is your age, i.e. A is the first digit of your age and B is the second digit...
"Multiply A by ten then add B" gives you the exact same answer as
"Multiply A by ten then add B then add six then subtract six" which is the same as
"Multiply A by ten then add six then add B then subtract six" which is the same as
"Multiply A by five then multiply again by two then add six then add B then subtract six" which is the same as
"Multiply A by five then add three then multiply by two then add B then subtract six".
