This roof is in serious need of replacement. Not only are the asphalt shingles losing granules it is also beginning to curl creating potential for leaks. Curling shingles are caused by a combination of age, hot temperatures and poor attic ventilation. If you want your shingles to last, make sure that you have lots of air ventilation. This allows the hot air that builds up in your attic to escape through roof vents. You can never have enough ventilation. If you are having your shingle roof replaced, arrange to have the old vents replaced with new high capacity vents. You may also want to add several vents as well if you have a large roof and a limited number of vents. Hot attics contribute to premature aging of the shingles and granular loss that greatly shortens the life span of the shingles.
Another situation that sometimes can contribute to premature aging is the effect of overhanging trees on your roof. The trees will prevent sunlight from drying the roof on damp days and they drop leaves and other material on the roof. All of this extra moisture can contributed to increased granual loss especially during extreme temperature fluctuations.
Try to keep your roof free of debris and also clean out any eaves troughs that you may have. Moisture can seep up under the shingles if the eaves troughs are plugged and full of water. Also avoid walking on the roof. Anytime you walk on a shingle roof, it will damage the shingles, causing more granual loss and in the case of the roof shown in the picture actually break off portions or at the minimum cause cracking of the shingles. Cracked shingle, loose shingles and broken shingles are perfect opportunities for water to seep in throght the roof causing more damage to the supporting structure underneath, the insulation and even into the living porting of the home.
Again, if your roof looks like the one in the picture above, have it replaced immediately before water causes even more damage inside your home.