Can I just replace the damaged section?

Often yes, and matching is the practical limit: shingles weather, and discontinued lines cannot be matched at all. A good contractor will tell you how visible the repair will be before doing it, and will sometimes suggest taking replacement shingles from a less visible slope.

Does a new roof need a permit?

In most jurisdictions in the United States and Canada, yes, and the requirements and inspection points are set locally. Ask who is pulling it — a contractor who suggests skipping it is asking you to own the consequence, because the permit is issued against the property.

Answers · Is something wrong?

How do I know if I need a new roof?

The short answer

Look for evidence rather than age: widespread curling or cupping, bare patches where the granules have gone, shingles cracking across the field rather than at one spot, daylight or staining visible in the attic, and a deck that feels soft underfoot. One damaged area is a repair. The same failure everywhere is a roof at the end of its life.

What actually indicates the end

Curling or cupping across whole slopes, not in one spot. Individual curled shingles near a vent are local; a field of them is age.
Granule loss over large areas, showing the darker mat underneath. Some granules in the gutter after a new roof is normal; bare patches are not.
Cracking across the field of the shingle rather than at nail lines or edges.
Repeated leaks in different places. One leak is a flashing detail; three in three places is the system.
A deck that feels spongy underfoot, or visible sagging between rafters seen from the ground at a low angle.
Daylight through the boards in the attic, or staining on the underside of the deck in more than one bay.

What does not, on its own

Age. Roofs in the same neighbourhood, of the same material, installed in the same year, reach the end at very different times depending on ventilation, sun exposure, tree cover and how well the original job was done. A contractor who leads with the age of the roof rather than with what they found on it is quoting from a database, not an inspection.

Repair or replace

The honest test is whether the failure is local or general, and whether the deck below is sound. Local failures on a roof with life left are repairs. General failures, or a compromised deck, are a replacement. A third case exists and is under-discussed: a sound roof with a bad detail — a valley, a chimney flashing, a poorly cut penetration — that has leaked for years and needs the detail rebuilt rather than the roof replaced.

Also asked

Can I just replace the damaged section?: Often yes, and matching is the practical limit: shingles weather, and discontinued lines cannot be matched at all. A good contractor will tell you how visible the repair will be before doing it, and will sometimes suggest taking replacement shingles from a less visible slope.
Does a new roof need a permit?: In most jurisdictions in the United States and Canada, yes, and the requirements and inspection points are set locally. Ask who is pulling it — a contractor who suggests skipping it is asking you to own the consequence, because the permit is issued against the property.

General information about roofs and about hiring, not advice about your building. Roofing contractor licensing, permits, insurance practice and deductible rules are set state by state and province by province across the United States and Canada, so your own authority is the current answer.