FFW, you are in a very similar situation to me, though I am ahead of you by three years. My eldest DS is 23,then I have DS 21, DD 17, DD14 and DS 6.
I do understand how you feel as I've felt very much the same, expecting those kinds of comments, wondering if I was being too selfish, but in the end, I realised that having that last baby, for me, was about being authentic and living with integrity. We are only once on this earth (that I know of, perhaps we do come back, but we're not conscious of it) so I have to do what is important to me. Nothing has given me the joy and satisfaction that mothering has, and some might say I am an addict, but I want to do it all again because I got (am getting) so much out of it.
I too feel that a sixth child will complete my family. And I know there will be grandchildren in the future,but I just don't see it as the same thing.
So, I know how you feel, we can do this together

The time between getting pregnant does seem to stretch out as we get older (for some of us). I too fell pregnant easily with my babies before my 40's ( first try every time). Charlie's pregnancy took 7 months of ttc, then after he was born my fertility took ages to return. Once it did, I think it was about 8 months ttc (lost Alfie at 10 weeks), then 6 months (lost Louie at 9 weeks), then not again for about a year (miscarriage at 6 weeks), then two years (chemical, lost at 4 week 3days). So, I guess I could be waiting a very long time now - or it will never happen. But on the other hand, maybe I will pop out a healthy egg any day? It's all such a mystery.
I like the idea that we are producing new stem cells for eggs all the time. How's that book review coming on Chewy? Do you have a comprehensive list of everything I need to do please??? I think I might even download the book today and have a peek myself. 
Ds and I looked after his cousins yesterday. He had such a wonderful time with his nearest in age cousin, who is about 10 months older than him. I remember well when my sister fell pregnant with him, accidentally, and how upset I was (happy for her of course, but so sad for me as I was desperate to have another). Well, turns out, I did get pregnant when I thought it would never happen, and the boys are great friends. It helps to remember that sometimes things do turn out for us.
And on that note, I'll say, have a lovely day everyone. Day 12 in my cycle, so time to start opks and think about shag-fest...Wish I felt more enthusiasm for it...