MSH,
dunno how to give you advise, cos this kinda of thing depends very much on your own mentality. some ppl chose to avoid all questions regarding why married so long still dun wan to have bb, some chose to let ppl know that they are trying but not easy, some just brush it off and said that they are not interested in kids, whichever way you chose to be, there is still this thorn in your heart. to your 50+ colleague, she could be so immune to it that this topic is non of her business anyway and she has no interest in it anymore, even if you asked her, whatever answer she gave you is not going to solve your problem, she and you have got different mentality towards this problem. Take one of our sisters here, BB, she will probably congrats her colleague and send her all her wishes and probably said something like: how i wish I could be you. But other like us might feel uncomfortable on this kind of situation. What I am trying to say is dun let this dwell on you, accept the fact that everyone of us is different, some of us can have things that others dun have, similarly I am sure you have things that your colleague dun have. When I was younger, me and my friends used to joke and asked each other: are you pregnant already and all of us will start to say: chor! phew phew, touch wood, i still have not enjoy enough yet.
Nowsaday when ppl asked me when am I goin to make bb, I will tell them : Okay,ok, soon soon, i go and plan soon and just smile. Usually they will stop asking further and then I think nothing of that question anymore becos nothing I can do about it.
It takes time and failure too, beginning is like roller coaster, mood is up and down and then after a while you get tired or resigned and start to plan for another route instead of concentrating on making bb. Dun let this matter bothers you too much, what will come will come, what is yours is yours, if it doesn't then it doesn't, you still can have a wonderful life, having kids is no easy task too.