The answer
Standard 3-tab shingles last 15-20 years; architectural shingles last 20-30 years. If over 80% of service life is reached with widespread curling/granule loss, replacement is needed. Per NRCA guidelines and building codes, if localized storm/physical damage exceeds 25%-30% of the roof surface area, a full replacement is required. Rotted decking requires full tear-off.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.