Perhaps we can suggest an alternative explanation. A human being is made up of two parts, the physical and the spiritual. These two components react differently to age. The physical body is in its prime only during its youth. As a person grows old his physical body weakens.
Old age is regarded as a liability to the physical component of the human being.
The converse is true with regard to the spiritual side of the human being. When one is young, he lacks a profound sense yiras shamayim. Only with age and life experience does one truly acquire deep insight into the ways of Hashem. Only with time does one truly witness the incredible hashgacha and mida k'neged mida of Hashem. With regard to this spiritual component of the human being, old age is an asset.