From what I've seen with mine, yes, they start to loose capacity as they get older and build the number of disch/chrg cycles. The most used ones I've had were for 18th scale and were 1500mah. After a little more than 100cycles thery were charging to only low 1200's after being run to 6.2v cutoff. When new, they would routinely see 1575, a drop of 20+%.

I assume IR goes up as well as they age, although not to the point with NiMhs.