The astrological explanation for why every 7 years there is a profound change in one's life is Saturn. The "7 Year Itch" is when Saturn forms a square moves approx 90 degrees every 7 years. But the profound effect it may have on one's life depends upon where Saturn is placed in the chart, its state, and what aspects it makes. Of course, aspecting itself is a big enough ordeal.
This "7 year change" occurs not only on the individual level, it also affects relationships, business, marriage, and anything else begun on a certain date.
In the life of a person, it is at about 7 years of age (first Saturn square to natal Saturn) when they begin to attend school, the first break away from the family. The first day of school can fill a child with fear as Saturn does affect us that way sometimes.