Even in 1963, they would have needed some sort of paperwork to establish identity when enrolling her into coal hill school... Perhaps the documentation for the identity of Susan foreman was appropriated from the Junkyard's office?
The description of running away in a clapped out old TARDIS the doctor used at the bum end of the 5 Doctors didn't describe the presence of the Master, Susan or the Stellar manipulator which can't have been acquired to destroy Skaro unless both the Doctor and Susan were playing it quite close to the chest in the Mutants... Which has me wonder if the whole little girl innocent role she adopted wasn't completely a... Ace was 16 and and she was mixing Nitro Nine to blow stuff up, Rose was 18 and acted more mature and on top of even the worst situations for the most part than Polly and Joe was in her 20s but acted like she as 9 years old.
I like the idea of counting in decades,
I also like the idea that that Galifreyans have trouble getting their heads around how tiny a human life span is that they have some trouble not accepting that a human who seems to be in his 400s is really only 22.