If you were twelve in 1972 you were born in 1960 which would make you 48 this year. Ok perhaps not until later in the year.
Both my wives (ex and current) are older than you ~ but then I was born in 1947 which, to save your aching brain means that I will be 61 this year.
No you are not doing your sums wrong. Acknowledged and legitimate children range from 40 down to 7 years of age. I shall say no more .... it is still too painful.