New try,

1900+10y-10x = decade of birth

2000+10x-10y = decade of birth

On equating both the equations we get:

y = 5+x

So possible candidates are in** (y,x)** = {(5,0),(6,1),(7,2)}

Now converting it to decade system. the solution becomes (6,1) (7,2) and (8,3)

(6,1) cannot be solution because then his age would be in between 71-79 in 2021 and no number between this range equals 7

(7,2) so his age would be between 71-79 in 2021, but only 72 is the age which equals 9, so it is our solution.

(8,3) so his age would be between 71-79 in 2021, but only age 74 is the age which equals 11, so we also include it in our solution.

So my guess is that it is 72 and 74.

so his year of birth can be either

2021-72 = **1949 **or

2021-74= **1947**.