Wizards coming of age

[finwitch]

No: HPFGUIDX 37683

Ali:
> I've just been idling away a bit of time wondering why wizards come 
> of age at 17 - not 18.

Well, my theory: it's because 17 is a Prime near enough 18.
Harry gets his letter at 11(also prime)
First time in Hogsmeade: Age of 13, 3rd year(primes)
OWLs at 5th year for 5 years of study (primes again)
NEWTs and adulthood, 7th year, at 17 (primes again).

17 over 19 because 9 is not a prime. It just fits in nicely.
29 knuts to a Sickle(prime again)... Primes are strongly associated 
with magical patterns...

Only exception to these Primes is the 4 houses, 4 Marauders and 9 
Weasleys. But... these a the squares of 2 and 3 - and thus, also 
considered magical numbers...