Literal Meaning

A body which is not yet aged, it is not yours

Implied Meaning

Before a person reaches old age, his body could be subjected to several hazards - illness, injuries etc - and end up causing an early death. These hazards, except the degenerative diseases which are common among the old,become reduced during old age,because old people are not very active so tend to move at a slow pace. Similarly a friendship can be interrupted by several factors during one's youth but if it is still in place by the time one reaches old age, it will be unshakable