OP, I had my first T 40, now 43 and due in three weeks. I can't actually believe I am 43, don't know were the time went, so in my head, I'm young. In my body though, I am very tired with this pregnancy, more so than the first but then I have a toddler to run about after. Like others, I think being older makes you more of a chilled out parent, more so then the rest of my NCT group, who seem to be embroiled in some form of competitive parenting! I'm too tired to get into all that.....
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It did take a while to conceive DC2 (one MC and chemical pregnancies, took a while to get a stayer) so I'd not plan it if I were you, just go for it and hope it happens. The ovaries can allegedly have a last hurrah so you may be blessed with twins as my mum was....just a word of caution!
I am the child of an older mother.....I will be the exact same age as my mum as she was when she had me and my brother. No parents living now, on either side so we have no help but are used to having no help. We are at an age where we see friend's parents beginning to need more support so are in a way fortunate that we don't have elderly parents and young children dependent on us.
Is it selfish? I don't think so, if you can provide a happy loving home and a sibling to current DCs, (I am one of five so wanted DD to have a sibling and shared childhood memories). I think you may regret what you don't do, rather then what you do. What does your DP/DH and other DCs think?