??? A complex way to explain a simple trick?

birth year+ age = now(year)!

now = 2020 (year)

1) birth year + age = 2020

2) birth year = 2020 - age

Assuming birth year >= 1900 AND birth year < 2000:

3) birth year - 1900 = (2020 - 1900) - age

4) 2 digits birth year = 120 - age

Couldn't be easier. Next year, use 121...