I was 41 (am 43 now) and i would say that the best age is really when you're ready - I'm just glad I was ready before my reproductive window closed!
If I had known how wonderful kids are, AND if my life had been completely different (regarding money, right partner(s), etc.), I would have started at 20 and had one every 5 years!
Since I did a lot of other things in my 20's and 30's - traveled, etc., I don't feel resentful at the sacrifices I am making now to be a parent. When my sister had her babies at 19 and 21, she was very jealous of my freedom and independence, because she never got to taste her own.