Every homeowner reaches this question eventually, usually standing in a driveway looking up at something that used to look fine. The honest answer is that it's rarely one obvious sign — it's a combination of a few factors working together.
Age Sets the Baseline
How old the roof is matters less on its own and more as context for everything else. A ten-year-old roof with a leak is a repair conversation. A roof past its expected lifespan with the same leak is a different conversation, because the leak is less likely to be the only issue and more likely to be the first one to show itself.
Where the Damage Is Matters
A single damaged section in one spot is usually a repair. Damage showing up in multiple, unrelated areas — a leak here, curling shingles there, granules collecting in the gutters — is a sign the whole roof is aging out at once, not that you got unlucky in one spot.
What You're Actually Solving For
A repair fixes what's currently broken. A replacement resets the clock. If you're planning to be in the house for years and the roof is already showing its age broadly, a repair can end up being the first of several, rather than a real fix. If the issue is contained and recent, repair is usually the right call — no reason to replace something that isn't done yet.
When in Doubt
This is exactly what an inspection is for — not a sales visit, just an honest look at what's actually going on up there before you decide anything. If you want a faster read first, the Oracle asks three quick questions and gives you a general sense of where things stand.