Then when mirrors were invented it was still considered a bad omen if the reflection was disturbed by a crack or scratch. The 7 years dates back to Roman times who believed the body renewed its self every 7 years.
You can reverse your bad luck if you bury the broken shards in the ground under the moonlight!...
Sources: http://www.irishcultureandcustoms.com/ACustom/Superstitions1.html
http://bit.ly/woYW7a http://www.brighthub.com/education/languages/articles/13474.aspx