Depending on the model of your toilet, there may be a rubber gasket on the underside of the hole where the flapper seats itself. In 7 years that could have easily gone bad. I suppose the spring that holds it closed could also have weakened. Check if there's a rebuild kit for your toilet make and model - the manufacturer's website might be a logical place to start. You'll surely end up pulling out the throne and rebuilding it - so super clean your black tank first or be prepared for major WHEWIE
when you unbolt it.
Of course if you're one of those millionaire Airstreamers you could just replace it with a new one that washes and blow dries your nether parts!
Your Avatar says you have a Classic - I'm assuming a porcelain bowl. If it's plastic it's probably time for a new one.