1)
What is the meaning of "Yenuvun"?
1.
Rashi: They will grow. Radak - in the days of Mashi'ach, Yisrael will live long - "chi'Ymei ha'Etz Yemei Ami" (Yeshayah 65:22). What is considered old in this world, will be like youth; they will grow and be fresh. "Ha'Na'ar Ben Me'ah Yamus" (ibid., 20) - when one will die at 100, they will say that he died in his youth.
2.
Malbim: They will have fruits in their old age, and have children.
2)
What is "Deshenim"?
1.
Rashi: It is fat.
2.
Malbim: They will be fresh with their bodily strength, that they can engage in Torah.
