Thanks for taking the time to post your thoughts!
I really don't have a problem with John and Sarah starting a family so late in their careers. I think that the producers needed to flesh out their personal lives a bit, and where Tom and Joyce had Cully and Joyce's poor cooking skills, John and Sarah have Betty and Paddy. To me, their family situation seems to be pretty normal - heck, my mom was in her 40s when I was born.
I think that in order to truly enjoy the show you have to allow for large amounts of poetic license. This is in order to accomodate the more than 300 murders and all the eccentric characters that we have come to expect.
My problem with the show is that they are only making 4-6 episodes per year.