I'm a quite newly switched user (February 2005), and pronouncing it Mac OS Exxx (Due to it sounding a thousand times better in Swedish, where Mac OS Ten or Mac OS Tio in swedish just makes it loose its flow).
Even due to this I've been aware of Mac OS since 94-95 or so (had a few basic computer usage classes in fifth grade, 11 years old), and I've got an old Powerbook 5300c somewhere.
I'm one of all those *Nix-switchers that basically just switched due to it's UNIX base, having used Linux as my main selection of OS since 97. Struggling with getting it to work decently as a Desktop OS, Mac OS X came as a saviour as it's the only UNIX-based OS worth to be called "Desktop OS".
Ehm, just rambling here... To the point:
Yes, I do know that OS 9 exists, and I don't pronounce it Mac OS EXXXX due to ignorance, rather for it being easier to say and having a certain flow to it.