Today is the feast of Pope St. Pius X, who sought to "renew all things in Christ". It was his belief and great devotion that "Holy Communion is the shortest and safest way to Heaven".
Before 1910, there existed no universally agreed upon age by which one may receive Holy Communion. Instead, the commonly shared principle was the necessity for a Catholic to have reached the "age of discretion" as described by St. Thomas Aquinas, the Fourth Lateran Council, and the Council of Trent. In practice, this principle led to a multitude of practices form diocese to diocese. For some, Holy Communion could be received upon reaching the age of 10, for others 14, etc.
Pope St. Pius X, ever the pastor, knew that frequent, even daily, communion serves as a remedy for human frailty and therefore instructed the Sacred Congregation of the Discipline of the Sacraments to decree 7 to be the age of discretion, though a child may receive even under the age of seven as long as the parents and his/her confessor trust the child understands well enough the great dignity of the Eucharist.
Today I was overjoyed to celebrate the first Mass of the new school year with the students of St. Paul School, my alma mater. May God bless the students, their teachers, and the staff.
Below is a section from Quam Singulari of August 8, 1910 by which the age of discretion was determined as seven.