I like this point from this Forbes article…”One of the reasons we engage in thinking about “old” the way we do is that we have no other terms for those ages above – say – 65. If we did have words for these latter stages of life, we would probably be better able to articulate where we and others are along that life continuum.”
Too often, this is associated with looks. But in the future as people stay active and engaged in life for a longer period of time, I think this might change. Staying engaged and active keeps us from becoming old.