To be honest, sooner or later every roof needs to be replaced, usually due to the long-term effects of weathering. If a residential roof is more than 20 years old, it is a prime candidate for reroofing.
To determine if you need a new roof:
On the ground, walk around your home with binoculars and inspect your roof for cracked, curled or missing shingles, as well as any excessive loss of the protective mineral granules. DO NOT CLIMB ON THE ROOF; walking on the roof is dangerous and can damage your roof.
In your attic, take a flashlight and look at the underside of the roof deck and rafters for any stains or wet spots indicating water leaks.
Call a certified roofer for a FREE estimate.