To put my post in context - I will be 36 when DS is 18 and so expect that he will have semi left home permanently by the time I am 40. (hopefully)
I think it's too old for some aspects like running around with them. I'm 22 and can run around with my DS like a mad loon that not even the 30 year old mums can do.
Having said that. I can see how young I am as a mum compared to the older mothers who seem to.. I don't know... get something else out of being a mother that I can't explain. Maybe it's the matured devotion? More time being able to be selfish (in the normal, natural way, not a bad trait way) and live life how you want and now are in a better place to give emotionally?
Physically I would not want to at all. I would want to conceive naturally if possible and not leave it to chance. I would want to be able to fling DS round my back with ease which the older mums just can't do. I want to be free to live life how I want in my 40s and wouldn't want to be tied to babysitters etc when I am in my 50s. I want to be a grandmother while I am still nice and young (though not too young, if DS is as young as me when I had a kid ). I also wouldn't want to build up a good career and then take time out to have a baby.
Emotionally though I can see the benefit of being much older. ATM I am very aware that I am still exploring who I am/ what I want and am not particularly settled in that way whilst trying to show a child the way in the world.
I also am trying to study while with a young child. The age I am, I think, makes this easier but it would have been easier to be sorted on that front before having kids.
I don't know. I personally wouldn't want to be anywhere near 40, but I also am aware that I was WAY too young.