The oldest i have is two years old come November I've re cloned it a few times and it's still good to go.
I've one i re potted in back February and I've just kept trimming it back, to clone from that just now would be less than ideal imo.
And another that i re potted at the same time but I've cut back to the wood regularly.
Lots of variables appear that affect the outcome, we'd all need to know all the fine details of the alleged dwindling genetics, not to mention the ability of the grower.
I've not found age to make any difference to vigor if I put the effort in before re cloning them.