Short answer: Hire a professional roofing company to assess your roof.
Age is a good indicator of how much life you have left in your roof but not always the determining factor (asphalt roofs typically last between 15-25yrs). Shingle brittleness, cracking, curling, and excessive shingle granule loss are the biggest indicators when determining how much life is left in the shingles. If the shingles are too stiff/brittle to perform repairs without damaging surrounding shingles, then it's time to bite the bullet and replace the roof. Also, if your roof is in need of too many repairs to count but the shingles are in overall good condition, then unfortunately a faulty installation job was performed. In this case, it may be better to replace the entire roof rather than spending thousands of dollars trying to undo what was done incorrectly to begin with and hoping further issues don't come up in the future.
Short answer: YES!
The older a roof gets the more attention it will need. That said, even if your roof is brand new it's a great idea to at least have it looked at on a yearly basis. If potential problems are found early, repairs are much less costly and you can have peace of mind during harsh weather conditions. Simply looking up at the roof from the ground or even from a ladder near the gutter will only provide a glimpse of your roof's overall health. We've seen animals cause a lot of roof damage, resulting in very expensive repairs that could have been minimized if found earlier. Tree branches that lay on the roof or break off from trees and fall on the roof have also caused roof damage, but most people aren't aware of problems until they see a water mark on their ceiling. Even a small roofing nail starting to back out of a shingle slowly over time, or something as small as one missing shingle can cause a bad leak, which can in turn cause rotted roof plywood, wet insulation, and sheetrock/ceiling damage.
I'm thinking of purchasing a home. Should the current seller of the house hire a contractor to put a new roof on, or should the new roof be chosen and contractor hired by me if I purchase the home?
Short answer: Buyer should hire and choose the roofer of their choice.
Although most homeowners who sell their homes want to do right by the new owners, choosing contractors carefully becomes increasingly more difficult for the sellers of the home. This is because the sellers need things done quickly so there are no excuses for potential buyers to rescind their offers. A problem with buying a home that was recently renovated is most new owners of the house don't know the contractors who performed the work or how to contact them. They may also not have any of the warranty information (if any was given) or know what materials were used. We have visited countless roofs for people who have no idea who installed their roof, and even if they do know, can't find a working phone number for them to help with roof problems they're currently experiencing. Although having a new roof, siding, gutters, HVAC unit, and paint job are fantastic, the shininess will quickly fade when problems start to arise and the repair costs start to mount. If the buyer desires all repair work be completed by the seller's hired contractors, it‘s important to get a list of the work performed, the contractors hired, and the contractor's contact information just in case any problems arise in the future.