Check out basic counting, 1-10, 11-20 etc. Also you're born at zero so technically everyones older than their current age
When you're 19 years old, you're still a teenager. When you're 20, you're not. QED.

But seriously, so long as the count of years is an arbitrary number originating from the estimated time of a particular historical/religious event, why not just go with the more aesthetically-pleasing value for milestones? As far as I'm concerned, there was a year 0, the millennium began in 2000, and we enter a new decade tonight. Everything else can stuff it.
More importantly, here's to a less fail-tastic 2010!