If you don't know your birthday you can still track your age by the passing years but you would certainly have had to guess or be told your age at some point when you were a child as very young children wouldn't understand the measurement of time.

It is common for some Cuban baseball players when they come to the US to obfuscate their age because they don't really have accurate birth records so sometimes they register as 3-5 years younger than they are estimated to actually be.

So my answer is if you aren't a Cuban baseball player trying to get a big MLB contract you probably think you are a year or two within the age you actually are but if you are a cuban baseball player trying to get a big MLB contract you think you are whatever age you can get away with that will benefit your chances of acquiring said contract.