Mathematical trick I discovered while playing around with my calculator:
If your next birthday is this year, enter your age in the next birthday, if it's next year, enter your current age, then add up the two digits of your age, this value will be A
Subtract A from the current year, as 2017, then subtract your birth year from the previous outcome.
The number you get from the previous step will be 9N, and N is the amount of full decades you have lived.
Basically, if you lived 1-9 years, the outcome is 9x0 = 0, 10-19 will be 9, and so on...