What’s That in Dog Years??*
Some 60 year-olds act (and look) like they’re 45; others, like they’re 75.
The same 
is true, kind of, for roofs.
While a typical roof lasts 20-25 years, there are plenty of exceptions to that rule — at both extremes.
Of course, it’s the roofs that wear out prematurely that concern home Buyers and Sellers (and their agents).
What causes that?
The list of culprits includes poor quality shingles, sloppy workmanship, extreme weather, and even overhanging trees (lots of shingle-eroding debris).
Regardless of the cause — or age of the roof — a Buyer who substantiates that the roof is not just old but failing is going to expect the Seller to either replace it, or discount the sales price accordingly.
*To convert “roof age” to “people age,” multiply by 4.
So, a 15 year-old roof is equivalent to a 60 year-old person.
