I had some 5-6 year old plants but they seemed to do best in the first couple years so I eventually gave up on them and started new. For me overwintering used to be very easy but now I basically give up due to constant aphid invasions
I overwintered two plants this past winter for the first time, aphids claimed one and the other is doing so-so, not particularly better than my plants started from seed. I don't think I'll overwinter again unless I get a plant putting out golden pods...