Well, if that were the only stipulation, then yes, you'd have a good point.
But it's not; don't worry, plenty of other idiots have asked the same dumb question without doing a modicum of research.
Not only does one have to be 19 years old, but they have to be one year of out high school. Which means, if your high school class graduated a month ago, in 2006, it'd be until 2007 before you entered the NBA Draft, even if you're 19 or older.
Now, the next question might be, "What if you've been out of high school for a year, but you're only 18? "And my answer would be, "I don't actually know, but I assume the age limit still stands."