Potentially, some of the service we perform in our own lives might be completed at age fifty. The Torah indicates as much when is discusses the parameters of the labor of the Levites. He will serve by giving advice. The Bartenura sees this Mishna as being directly interpretive of the verse in Parshat Naso. Taking comments of Rashi and Bartenura together, it seems that the ages chosen for the service of the Levites in the desert were reported to us because of their universal nature.
People develop in stages, and they change.
Twenty-five is different than fifty etc. The Torah indicates that these changed represent a kind of law of personality development, which is important to take note of. Once noted, it will help us in life to determine which students to teach and from which people to accept advice. Gra in his comment to the Mishna points out a Gemara, Chagiga 14a. The prophet says that we will lose our leadership as a punishment.