My MacBook was purchased in 2008. Yesterday I checked the number of reallocated sectors of the hard drive using Disk Utility. It showed there were no reallocated sectors. That seems strange since I haven't replaced my hard drive since 2008. Can I rely on Disk Utility for this information or I need to download additional software?
Typically, I don't see any relocated sectors for years, then dozens appear at the same time when the disk is about to fail.
I tend to demote disks to being used as backup devices or file transfer disks in an external enclosure after 3 years of active use because that's around the time I find they usually start to fail.
Reallocated sectors frequently (but not always) appear when a hard drive stars to fail. Just a few back-of-the-envelope calculations here:
If you bought your computer in the beginning of 2008 and used it 24 hours a day, you'd have used about 32,000 hours of hard drive time. I don't know what model of hard drive your computer has, but the mean time between failures for not-pariticularly-impressive hard drives from that era is about 300,000 hours. So in the heaviest-use scenario, you've used your hard drive for about 10% of its expected life.
I don't think there's anything strange about a lack of failed sectors at this point.