Is 117 something like a magic number?

Question

Why is it that if you add the last 2 digits of your birth year + your age or the age you will become on any particular year it always adds up to 117? For example, I was born in 1975 and turned 42 this year so: 75+42=117
My dad was born on 1947 and turned 70 this year, so: 47+70=117.
My brother was born on 1985 and turned 32 this year: 85+32=117 Why is it always 117 for anyone born in the 1900's?

Answer 1

It is now $2017$, which is $1900+117$. If you were born in year $y$ of the $1900$s the last two digits of your birth year is $y-1900$ and your age now is $2017-y$. Adding these gives $117$

Answer 2

The definition of age (as long as you've had your birthday this year) is $$ \text{Age} = \text{Current year} - \text{Birth year} $$ Rearranging, we get $$ \text{Age} + \text{Birth year} = \text{Current year} $$ which is what you have observed. You're off by $1900$ (i.e. get $117$ instead of $2017$) because you're using only the last two digits of your birth year, which for people born in the twentieth century in practice means that you've subtracted $1900$ from the left-hand side of the last equation.