From a medical standpoint, before 35. This way, you should still be young enough to be sufficiently fertile, and not so old that you are at increased risk for birth defects. I understand why people would give answers about financial and mental stability, but I don't think that is the nature of this particular question. However, I'll add my two cents... The average age nowadays for having a first child is 25, and I think that's a pretty good age. You've had a few years of experience as an adult psychologically speaking, and should be at your peak physically speaking as far as conceiving and carrying a child (and being energetic enough to run after and play with and care for the kid after it's born). That said, everyone's different, so these numbers may or may not be best for a given individual. All financial advice is to be taken with a grain of salt, as "financial stability" is something of a myth (you could hold down the greatest job for a decade, get preggo and then lose your job a month later, for all anyone knows), so if you want a kid and feel 'as ready as you'll ever be' (no one is ever 100% ready to be a parent)' then go for it.
As long as you are healthy, financially independent and can take care of yourself. Also, having all your education behind you is a good idea. I had friends in college who were raising kids and they would tell me about arranging for babysitters and would even bring their kids to the class if they could not find a sitter.