I'm in my twenties. Sometimes I forget my age. Only ever by one year -- I just don't remember if my birthday passed or not. Kind of like in January when write oit today's date and accidentally use the previous year. After a few mistakes, your brain adjusts and you start using the right one. Same thing for my age, except that I don't actually think about my age very often; it hasn't mattered much since I passed 21.
A colleague of a old friend would recall your birth year (and date?) to know your age. It is a value about you that doesn’t change and then you can derive the age.
I’ve started doing similar now. I don’t often recall my age or the number of years my wife and I have been married, or, heck, even our kid’s ages. But I recall all the years and dates. In this way, I more easily appear to recall the right value.
How did they manage this? I've tried every way I can think of forgetting my age, to no avail -the bastard number is always there :)