It's very hard to wear out a front tyre on a pedal bike; it's usually cuts and punctures damaging the casing that finish them off.
After several years, depending on where the bike has been stored and the quality of the tyre, the rubber content will perish causing the tyre to stiffen and crack. (If you left a brand new bike outside for 10 years without it moving, the tyres would be shot even though they'd covered 0 miles.) This is the reasoning behind swapping the front and rear tyres over periodically - no one wants to throw away a tyre with lots of tread left because it has partially perished.