Sonic's birthday is the date of Sonic the Hedgehog's birth. Sonic's canon age is fifteen years old.
In the games
At the start of Sonic Generations, Tails and Sonic's friends throw a surprise birthday party for Sonic. Sonic is given a chili dog as a present from Tails, but when the Time Eater appears it gets blown away (much to his disappointment). Later on, the party continues when the Time Eater is defeated and Sonic gets back his chili dog, which is still warm.
How old Sonic became is never specified during the event.
In the cartoons
For a gift, Tails builds Sonic a flying bike. Realizing Tails' potential with mechanics, Sonic asks Professor Von Schlemmer to make Tails his research assistant, but then Robotnik captures and impersonates him to get Tails to build his latest anti-Sonic weapon.
Sonic has another birthday party in this episode, with Tails getting some of Sonic's friends for a party.
Sonic shares his birth date with his sister Sonia and his brother Manic. They were all born on March 4. This is coincidentally the same date when the portal to Mobodoon opens every four years, Mobodoon being the place of the triplets birth.