As far as I understand, the reason their life span is shorter is generally because their heart can't cope with the size of the body, given the heart is the same size as that of a smaller rabbit. So, does that mean that the general 'wear and tear' on the body is actually the same as an average 5 year old bunny- so just into middle aged; I don't know.
I would tend to advise more on looking at the condition of the bunny, than an age, personally. I've never needed to put an older bunny on older food though, if anything they tend to need to go back to junior food.