When is it time to replace roof shingles?

Hi all,
I have some shingles (not sure what type) on the house I just bought. The shingles are about 13 years old. Some of them are starting to curl up a bit. I'm wondering, how to you know exactly when you should be replacing the shingles?
Thanks for any info!

In addition to what Larry and doug said, it may be that you have a 15 year shingle on your roof and they are approaching their max life. Start planning now using what the other guys have saide with with regard to ventilation and color. Color has been sown to be one of the biggest factors in reflecting/ absorbing heat toward the interior. Also, if you find that you have adequate soffit and ridge ventilation you might also consider a powered ventilating fan in the roof. It made an amazing difference in my house.
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bit.

It's probably a good idea to start researching contractors way ahead of time. Ask friends or co-workers if they've used someone they were pleased with. The last thing you want to do is be rushed into a decision when your roof is leaking. Look for the signs that contractors put in the yard when they work on homes, and maybe ask the customers about the work when it's finished. Check names with the BBB.
Some less obvious things to check with customers, or ask contractors: Did they take whatever measures possible to protect shrubs from falling debris? Are they equipped to cover partially finished areas of the roof in case of rain? How do they handle the removal of nails on the ground? (After my roof was done, I found exactly one nail. The contractor dragged a large magnet all over the place). Do they carefully inspect roof edges and make recommendations to improve drainage relative to the gutters? If ice damming is a potential issue where you live, and you get even a little ice buildup in your gutters during the winter, you MUST focus on that when talking to contractors. Sometimes it's not possible to improve the situation, but they can avoid making it worse. If they've never seen your gutters in the winter, you need to tell them of problems in specific spots.
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I'm surprised the inspector didn't discuss this with you before you bought the house? Did he get up on the roof to check it?
One sign of need for replacement is brittleness. You'll begin to see crumbling during high winds. Shingles should be flexible when they're in good shape.

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What Doug said, plus this: when shingles curl prematurely it is usually a sign of poor ventilation under the sheathing. There needs to be airflow from the soffits to the peak of the roof between the sheathing and the insulation. If this doesn't occur, then you will constantly be replacing shingles. Fixing the problem can range from very easy to extremely difficult and expensive depending upon the type of construction.
If you can't fix the ventilation problem, probably the next best thing is to use very light colored roofing - preferably white as it won't absorb as much of the sun's rays raising the temperature of the shingles.
FWIW,

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Shingles should be replaced 1 week before the roof would start to leak.
Curling edges is a sign that it is starting to wear. Brittleness is another factor. If you can get on the room and try to bend one of those corners you will be able to tell.
Depending on the grade, climate, attic ventilation, 13 years is not all that old. Most builders grade shingles will last 15 to 20 years. I replaced mine and 23 years and had lo leaks. YMMV quite a bit, of course.
One more thing to keep in mind. it is usually permissible to have two layers of shingles on a roof. If the edges are curling though, you want them removed before putting on the new roof or you will have problems.