MM is quite good with giving ages and DOB's of characters, we know Tom Barnaby was born in 1943, same as the actor. I guess John Barnaby his cousin was born about 1965. Also several ages of the guest characters have been mentioned. Would the profiles for the guesties have room for their approximate DOB, such as Muriel Harrop was 80 in Blue Herrings, shown 2000 (but probably set 1999), so she was born about 1919, as I think many eps are set months before transmission date.
I am a bit of a buff when it comes to ages of characters in TV shows, I also love Corrie, Emmerdale Aussie TV show Neighbours.