How are you supposed to know when it is time to replace aging roof shingles? The first thing to realize is replacing shingles depends on their overall condition, and not just their appearance, or just one or two leaks occasionally.
Asphalt shingles are the most common used on roofs. They are manufactured by saturating organic felt mats or Fiberglas mats with asphalt and then covering the top surface with little mineral speckles. The speckles give color to the shingles and also protect them from the sun. Most of these shingles have a life span of about 20 years. More specific life depends on weight and how much sun the shingles get. So southerly exposed roofs get more sun and therefore deteriorate the fastest on rooftops.
As shingles get old, they become brittle and their corners start to curl. This makes them vulnerable to wind damage to the point where they can blow away. Shingles can also get surface cracks and pitting when they start to lose their speckles, or granules. This can lead to mat exposure, and then leaks. The time to replace your roof shingles comes down to how much deterioration is detected, and to what extent, as discussed above. If over 25% to 30% of your roof shingles are deteriorated, then its probably time to replace your roof!