Yeah I agree. Mine are nearly 8 and are still on the standard SS. I tried the mature for a while but found that they lost weight.
However I think Louise and Gus uses it and hers love it and thrive on it, so I reckon it very much depends on the individual bunnies. I think its content is better - different sources of fibre, less alfalfa etc, so might be useful especially if the buns are not good hay eaters and/or have slightly dodgy poos. But personally I wouldn't rush to change unless there is a particular need to do so.