Generally, a very small (even microscopic) scratch, dust can cause bad sectors: then the affected surface area may not be read/written.
Hard disks in general detect such condition and automatically try to avoid using that sector and use the spare area instead for all reads/writes.
So having relatively small number of such bad sectors may be accepted (see
http://www.hdsentinel.com/faq.php#health )
The most common cause of such issues are mechanical damage / shock which can cause damages inside the hard disk.
Also frequent parking, power cycles can cause minor material cut-off - which result in damaged sectors. Usually yes, these may occur after long time use, usually after many years.
And in some cases it is possible that the problems were present on the hard disk since installation, just as the issues are on unused sectors, they may remain unnoticeable for long time, maybe for years, until that area is used.
For example,
http://www.hdsentinel.com/hard_disk_cas ... ectors.php page describes such situation, when the problems were hidden until the hard disk is filled.
This is why it may be important to test the drive (even a new hard disk drive) carefully before using for storage, as described at
http://www.hdsentinel.com/faq.php#tests