Honestly, aging for monsters is out of scope for Pathfinder. You encounter monsters, you don't create them or keep them around for long periods of time until they die of old age. Heck, campaigns aren't meant to run that long, and certainly racial life spans aren't even enough to encourage such a thing.
Like if you had a human, dwarf, elf and a Shabati in a party and they all started as 'young' by the time the elf hit old age the human and dwarf are long dead without some sort of magical support. And the Shabati is immortal.
Age is usually reflected in monsters by altering their stats or adding a template. Individual monsters definitely do vary. Like all Outsiders are default immortal, but that is a default. Dragons are a huge exception because they improve stats with age.
So really, there are no easy answers for the OPs question.
I think the standard ageing penalties/bonuses are set from the perspective of the standard player races.
Monsters who naturally have ability scores much higher than humans or elves or dwarves or gnomes should perhaps take proportionally larger physical penalties - or not, as their physiology may change in ways completely different to what's normal for a human (as already mentioned with dragons).
|David knott 242|
Animals should definitely not improve their intelligence scores with age. Many reptiles continue growing indefinitely with age, so I doubt that they would ever lose strength while they are alive.
I think I would limit PC-like aging to creatures with life cycles similar to typical humanoids.
For the purposes of sands of time, if the creature ages normally, they age normally, if not, they take damage. They call out undead and constructs as not getting older, so that's straight forward. The creature also does not gain the benefits of aging, so we don't need to worry about smartening up animals with age.
As a simple rule of thumb, anything that has a racial immunity to death effects or critical hits probably doesn't age.
I believe that if you look at the monster type information pages (where they say eg all outsiders have dark vision etc) then certain creature types say they do not age. Which would imply all the others do at least age somewhat. At that point how long their life spans are is not really known except in maybe a few creatures description block.