Tavares was drafted as a 14 year old (May 05 Draft, September 1990 birthday), but I believe he turned 15 about 4 days before Oshawa's first actual regular season game. Other recent 15 year olds to play in the OHL include Rico Fata and Jason Spezza, who both played under the old rule that allowed exceptional 15 year olds to play for their hometown team, before entering the Priority Selection at the conclusion of the season.
The rule now allows 15 year olds to apply for exceptional entrance (like Tavares), for a fee of $1 000, plus they have to gain approval by an appointed board. John McFarland is the only other known player to try this, in 2007, but was denied.
Players in the WHL can play at Age 15, depending if they are called up, and I believe once their club season is over they're allowed to play games with the WHL team. Mark Pysyk played 14 games or so in his 15 year old season a couple years ago.