While it made sense that Jamie would send Claire back through the stones since he's bound to die at Culloden, really DG missed a trick by depriving her 2 main characters of 20 years of bonking during their sexual peak
Initially I thought so too, but as I kept reading I actually started to love their middle-aged sex life. In many ways they have greater intimacy and fewer inhibitions. Not to mention the fact that if they'd been sexual partners during Claire's childbearing years, she would have been constantly pregnant and breastfeeding (in other words she'd have been Brianna, and we know how well that turned out!) so nowhere near as interesting.
I also found the reference to having children ridiculous given her age. In one of the later books, after a particularly harrowing ordeal, Jamie actively considers that possibility - I think by then Claire would be in her mid-fifties!
I found that the best way to address the age question is simply to more or less ignore it and not try to get too precise. They reunite when they're both in their mid-forties, more or less :)
I was watching my dance teacher yesterday, who is I think at least 50, has wild curly brown hair, beautiful blue eyes and an amazing body, and i thought - yep, totally believable!