While there's no number that would automatically mean readiness (just like being 18 doesn't automatically make one an adult in mind, though too many kids who are 18 are offended if you don't think of them as mature adults), I absolutely would not support anyone under 22 trying to have a baby. I don't care that almost everyone under the sun who is 18, 19, 20, thinks they're soooo mature and ready. Every person chances a LOT between the late-teens and mid-20's, and it really is immaturity that blinds a person from being willing to learn from those older than them. When you were 14, you probably thought you were mature, but look at how much you've changed and how immature you now see that you really were. That was only a few short years ago.
As for over 22, I'm not going to automatically be all, "Yay! You'll be an awesome mom!" No. At this point, its about had the person thought it through? Is the relationship truly stable? Is there a plan to support the baby that is NOT tax dollars? Have the couple thought about things such as what happens if one loses a job or even dies? What is the reason for wanting a baby?
I fully expect someone to take offense, but I'm not going to lie and say, 'Oh, as soon as you turn 18, it's all good!" because that would be lying. I have seen far too many teen mothers (and I am the ONLY one in my ENTIRE family EVER to not be a teen mom among those who had kids - and only a couple didn't), and I've seen the reality of what they go through.
Having a baby is not a game, and being truly ready means more than just wanting one and having a job at the moment.