'Old' in the Bible

Ishbosheth Saul's son was forty years old when he began to reign over Israel, and he reigned two years. However, the house of Judah followed David.

And Jonathan Saul's son had a son that was lame of his feet. He was five years old when the news came of Saul and Jonathan out of Jizreel; and his nurse took him up and fled. And it came to pass, as she made haste to flee, that he fell, and became lame. And his name was Mephibosheth.

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

And Barzillai was very aged, eighty years old; and it was he that had maintained the king while he abode at Mahanaim; for he was a very great man.

I am this day eighty years old: can I discern between good and bad? can thy servant taste what I eat and what I drink? can I hear any more the voice of singing men and singing women? and why should thy servant be yet a burden to my lord the king?

And she spoke saying, They were wont to speak in old time saying, Just inquire in Abel; and so they ended.