Yeah, i reckon it's different for everyone, lots of men out there who are parents at the age of 50+. For me I feel more sad that I will probably be a housebound old crusty biddy by the time my son is my age but you never know, maybe I'll be able to be mobile and energetic into my 80's. I hope so, it certainly inspires me to stay fit and eat well.
Cormac McCarthy wrote his book 'The Road' about a man preparing his son for his (the father's) eventual death in a post apocalyptic world (cheery I know). He was 70 when he wrote it and had a 10 yr old son at the time which inspired the story. Great book, not for the faint hearted though. Makes me cry every time I read it.
I think if I was still trying for my first I would draw the line at....hm....I'd say 45 but you look at Virginia Trioli (ABC news presenter) and she had her first at 47 so I'd say (personally) 48? As to what I would ACTUALLY do in that circumstance I couldn't say. In seven years time I might find the idea of having a baby totally insane.