'Old' in the Bible

Ish-Bosheth the son of Saul [was] forty years old when he became king over Israel and he reigned two years; however, the house of Judah {followed} David.

(Now Jonathan the son of Saul had a son who [was] crippled in the feet. He [was] five years old when the message of Saul and Jonathan came from Jezreel, and his nurse had picked him up and fled. It happened that as she [was] hurrying away to flee, he fell and became crippled. His name [was] Mephibosheth.)

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

Now Barzillai [was] very old, {eighty years old}. Now he had provided the king [with food] while he [was] staying at Mahanaim, for he [was] a very wealthy man.

I [am] eighty years old today. Can I discern between good and bad? Or can your servant taste what I eat or what I drink? Or can I still hear the voice of singing men and women? Why should your servant be a burden any longer to my lord the king?