I'd say go for it - see if it happens.
My dad was 50 when I was born and I always felt proud because I knew he was old, but he had a lot more go in him than a lot of young men. My brother (3rd bro) was born when my dad was 60. And I'm glad to have my siblings! My dad was 30 yrs older than my mum who always seemed very old to me.
The thing I think is more important than years already spent, is life expectancy, and of course your mind set (some young people seem so old, whereas some older people seem very young). So, go for it, give your DC a sibling if you can. OK, some siblings don't get on. But, I'd say a lot really value having siblings and they're usually there long after the parents.
Finally, I had two kids at 32 and 34, and just had my 3rd at 40. I wont be having any more, because the birth was very risky and I want to be here for my kids. But, that was not age-related. My DD2 is perfect. If I'd not had such a tricky post-labour, I'd consider having one more at 41/42. I don't think it's unfair, because the kids have each other and like I said, we just don't know how long we have left, no matter how old we are when we have our kids.