Chains should be replaced before they stretch. Get a ruler and place it along the bottom of the chain on the bike (taut.) Measure a 12" segment from center of rivet to center of rivet. if you have more than 1/16" its time for a replacement. Replacing the chain before it stretches too much extends the life of the cogs and chainrings.

Cogs last a longer while than chains. Chainrings can last many many years. The larger the cog/ring the longer they last, smaller ones wear faster. Shark fin teeth are a sign of worn rings or cogs (too late) and will damage any new chain

Rear tires wear out faster than fronts, but don't just replace the rear; your best tire should always go front because it bears the brunt of tracking and traction needs