In America, yes. In the UK, no (although it is getting more common with younger people I find)

I heard a theory that it’s because Americans have $100 bills, whereas the UK only goes up to £50. So counting up in hundreds is more natural in America

Yes, technically it’s ephebophilia. Also, fuck you! Don’t be having sex with minors!

To be fair, it’s not like we were saying “one thousand, nine hundred and ninety-nine”