Musar על שבת 36:9

The plain meaning of the verses dealing with the שמטה and יובל legislation points to the need for the seventh year to enable the land to assume a spiritual dimension after six years of continuous farming just as the dimension of time is consecrated on the Sabbath after six days of toil involving mostly mundane, secular matters. Looked at from an allegorical point of view, we may think in terms of a lifespan comprising seventy years. These years, commencing at the age of twenty and coincidental with full accountability [for one's actions] to Heaven, are considered the fifty years of adulthood. The seventy years which comprise an average lifespan allude to the seven days of Creation during which G–d issued ten directives (daily?). When you deduct twenty from this total you are left with fifty, i.e. seven times seven.