I'd forgotten about Young Sam, his various appearances mean that Thud and Snuffare several years apart - probably 3 or 4?
I'd dismissed the the reference to the Dean's age and hence Ridcully's as ''one of those Discworld things''. Ridcully is a vigorous, active man eating huge meals and carrying out a demanding job, and so unless wizards age at a different rate, he cannot be over 80 at the time of Unseen Academicals - in which, after all, he plays football. Windle Poons seems to suggest that wizards age much as those outside the walls do, but it's implied that time within the faculty cannot be relied upon.
Colon must be older than Vimes or Nobby, but close enough that he served with Nobby as an infantryman at a time when Nobby seems to have been sufficiently experienced to have had some sort or rank in the Q branch ( allowing for Colon exaggerating somewhat, in the manner of old soldiers everywhere ). He also worked quite hard in Jingo ( on the pedals in the Boat )
On all that basis, the overall ages at the time of Snuff - Ridcully in his late 60s approaching 70, his brother Hughnon a few years younger, Colon about the same, Vetinari in his later fifties, Vimes three years or so younger and Nobby five or six years less than that - seems about right, given the various contradictions and inconsistencies.
Angua could well be older than Carrot, she certainly seems to have more of a back-story. Do werewolves age slower than humans? We don't know, but since wolves don't, rather the reverse, the inference has to be that they don't.
The witches' timeline varies greatly as the plot demands. On a general basis, I'd say that Nanny Ogg is about 70 at the time of Snuff, and Granny Weatherwax somewhat less - being close enough to have had a youthful relationship of sorts with Ridcully. It could well be that, growing up in a place like Lancre, they could be very close without actually being the same age. Likewise although the age of majority seems to be 16, I wouldn't be much surprised if Nanny was married at 13 or 14.
Ponder Stibbons being generally early 20s to just past 30 seems about right, given his character and various plot points he is required to cover.
Susan and Mort don't really fit, but their status as sometimes resident in Death's Domain - and Susan's apparent status as semi-human, although her parents were undoubtedly human - mean they don't have to. Reaper Man
is another book which doesn't fit, but doesn't really need to, as is Pyramids.
So, it's fairly easy to compile a general timeline which doesn't bear detailed scrutiny, but no more.