The oldest pot plant in the world is a very large (14 foot long - its trunk is horizontal) Encephalartos altensteinii. It was only when I started this post that I realised, somewhat to my surprise, I'd never showed it to you. That plant lives at Kew gardens (which I've visited (and shared pictures from) on any number of occasions since I started this blog), is at least 250 years old and will have to wait for next time.
This E. altensteinii is a mere pup compared to that one and lives at San Diego Zoo. Notwithstanding its junior status it has at least pushed out a male cone (the Kew plant has coned once in the 227 years since it arrived there).