The easiest way to find out the age of your roof is to ask the previous homeowner or contact the company that installed the roof. You can also try looking through your closing documents from when you purchased the home. Sometimes roof warranty information is included.
If that information isn't available, you can find signs that your roof is older. Look for missing granules, curled shingles and dark spots on your roof. If you find them, your roof has likely been in place for at least five years.