Commentaire sur Le Lévitique 15:16

ואיש כי תצא ממנו שכבת זרע, “and if a flow of seminal emission originated from a man;” the word איש in this verse exempts a minor from this legislation, as long as that minor is not yet nine years of age and one day.

כי תצא ממנו, as long as this semen has not left his body it does not confer ritual impurity. Seeing that this sounds somewhat superfluous, the Rabbis interpreted this to mean that both a priest and an ordinary person who experience an erection can or should (as the case may be) take hold of their member to prevent the emission until it is not likely to come into contact with something subject to contamination. (Sifra)