My partner wanted to have a child because she was sure that a life/couple with a child is better than one without, no question. I never wanted a child, but in the spirit of promise of love to her, to make her happy, I decided to take the plunge. The sacrifices are many, and probably greater than expected, but the net result is that I think that the sacrifices, once you begin to make them and not dwell upon them, seem trivial, and the gains incomparable.
My wife and child have been good for me spiritually, and this I hope to value above everthing else now. I'm arrogant enough to think that I could have done some things as well being single, but I'm not sure that that's true. In the end, I agree with her -- in my present opinion, a couple with a child being raised well is better than one without. But this is not to say that a selfishly motivated couple ought to have children for the sake of appearances, social pressure, etc.; to the contrary.
We married and procreated late -- around our 40s. Our child is healthy, intelligent, intuitive, happy, and (overally, usually) good (emphasis for value). Health, happiness, etc. are certainly blessings, but I don't think you can count on them. You have to be prepared to have whatever sort of child appears, and adjust accordingly. We have no regrets about our ages in this; we have even older parents in our circle and we find them happy well into their retirement with their child just entering full adulthood.
I suppose it makes sense to maximize your time with your children and therefore have them early, but if you have to trade off your own preparedness, maturity, stability and attention towards having children earlier (esp. e.g. with the wrong partner or partner's frame of mind), then of course having children later can be better than ealier in some cases. But beware that you cannot exhaust all your desires by exercising them; having a child because "the biological clock went off" is probably among the poorer reasons; the best learning is by doing.
Why have children? Should you have children? Look into your hearts.