If wikipedia is accurate *tosses a coin* then Hershey was about 37 when she became Chief Judge and had the job for about 9 years. Uma is what? Late 30's? Early 40's? So she's probably about right.
Mind you I think the script describes the unnamed Chief Judge as being in her 50's, so if anything Uma and - more to the point - Hershey is a little young. Perhaps this is supposed to be McGruder after all? Or perhaps no established Judge in particular.
As for Dredd's age; I think there's something about him ageing slower due to his being a clone and using the sleep machine so much, so they can always fudge it a bit. That or Fargo was a Dúnedain.