Rashi on Numbers 4:2/רש"י על במדבר ד׳:ב׳

1נשא את ראש וגו׳ TAKE THE SUM OF [THE SONS OF KOHATH] — Count those that are fit among them for the service of carrying, and these are the men from thirty to fifty years, for a person less than thirty has not yet attained his full strength, — hence they (the Rabbis) said (Avot 5:21) “a man of thirty has reached the age when he enters into full strength”, — as for one who is older than fifty, his strength gradually diminishes from then.