'Old' in the Bible

Ish-bosheth son of Saul was forty years old when he began to rule over Israel. He ruled two years. However, the people of Judah followed David.

Now Saul's son Jonathan had a son who was crippled in both feet. He was five years old when the news about Saul and Jonathan arrived from Jezreel. His nurse picked him up and fled, but in her haste to get away, he fell and was injured. Mephibosheth was his name.

David was thirty years old when he began to reign and he reigned for forty years.

But Barzillai was very old -- eighty years old, in fact -- and he had taken care of the king when he stayed in Mahanaim, for he was a very rich man.

I am presently eighty years old. Am I able to discern good and bad? Can I taste what I eat and drink? Am I still able to hear the voices of male and female singers? Why should I continue to be a burden to my lord the king?