What age do you think is good to 'settle down'?

So I have recently turned 27 and am starting a PhD which will take me 3 years to complete. I am in a long term, stable relationship. My partner has a good career and salary which looks set to increase. He is already 31. I would rather put off any further developments in our relationship (marriage, kids etc) until I have completed my PhD- which he is happy to wait for. We are not ready to for marriage at the moment, but probably will be in a sooner time scale than 3 years if it was not for my PhD.
I will be probably 31 by the time we get married and I guess 32 or 33 by the time we start thinking about our first child and he will be even older. While I don't think this is too old, I know that things don't always run exactly to plan (e.g. if we split up, had financial difficulties, if I had problems conceiving etc etc) Do you think that I am leaving things too late? Marriage while completing my qualification would not be out of the question, but I would rather wait until the stress of it is over!
Thanks for your help.