If I was born in 1996 in 2017 i am 21 years old.
From year 1996 to year 2006 there are 10 years.
From year 2006 to year 2016 there are 10 years.
-> From year 1996 to year 2016 there are 10 + 10= 20 years.
And from year 2016 to year 2017 there is 1 year.
So, 10+10+1=21 years will pass from 1996 till 2017.
