Rashi on Numbers 3:40/רש"י על במדבר ג׳:מ׳

1פקד כל בכר זכר … מבן חדש ומעלה NUMBER ALL THE FIRSTBORN OF THE MALES [OF THE CHILDREN OF ISRAEL] FROM A MONTH OLD AND UPWARDS — as soon as he (the firstborn) has passed the age when it might possibly prove non-viable (Numbers Rabbah 4:3; cf. Rashi on v. 15).