The weird thing is it hasn't even been that long. The dwarves in question were 56 and 57s and it has only been 5 years since embark (they are now 61 and 62).
Even the children get progressively wimpier as they get older. Some of the babies started out as agile and mighty and incredibly tough. I can load the save back when they were born and they have those attributes. Loading the more recent save with them being 3 and 4 years old suddenly they are losing their good attributes and gaining bad ones, like the agile and incredibly tough one is now very weak and extremely quick to tire. Another that was average (no modifying traits) has become clumsy, very quick to tire, and unquestionably weak.
Skills seem to be rusting a bit quickly too. My doctor, who has treated quite a few wounds over the years from goblin attacks, is getting worse and worse instead of learning more. He began as adequate wound dresser, novice diagnostician, adequate surgeon, competent bone doctor, and adequate suturer. Now hes novice, dabbling, novice, adequate, novice. He has treated my 5 military dwarves at least 30 times over the years with a few of them getting minor wounds at every goblin ambush (usually minor, a quick clean/suture/dressing and they're up and about again), yet hes still losing his skills? What do I need to do to train a doctor, build a trap for my own dwarves to drop them a few z-levels every week and have a squad of dedicated test dummies for doctor training?