At the end of Unseen Academicals (or, if it's easier to place, Thud!), how old is Sam Vimes? Is it possible to even figure out what his age is?
We see Vimes in his forties to fifties, but his exact age is not given out in a consistent way. Terry Pratchett has stated that each book happens on a slightly different leg of the Trousers of Time: “There are no inconsistencies in the Discworld books; ocassionally, however, there are alternate pasts.”.
Night Watch has uncharacteristically precise indications about Vimes's age, what with
Vimes traveling through time and meeting his earlier self
Night Watch opens and closes on the 30th anniversary of the Glorious Revolution of the Twenty-Fifth of May. At the time of the revolution, Vimes is a rookie recruit to the Watch; I seem to remember his age being stated as 16, and in any case in his late teens. That would make him 46 when Night Watch ends and his son is born, and therefore 47 or 48 in Thud!.
Other Watch books make him a little older than that. In fact, even Thud! states that Vimes was a blackboard monitor “in that little street school more than forty-five years ago” and that he was 6 years old at the time, hence he would have been at least 51 in Thud!. I seem to remember Vimes being fiftyish in earlier books, though I can't find a quote to back it up at the moment.