Whether 16 years old, or 17 years old or 15 years and 364 days old is the right age at which to set consent is, yes, somewhat arbitrary but also a necessary legal decision. The difference is that, if the legal age is 17 and a 16 year old has sex, a law may have been broken, along with other potential consequences, but it is not a like with like comparison in any way other than the setting of an arbitrary time frame based on a best guess. Where they are very different is that in the case of abortion if that best guess is wrong, even by a second, or a day, or a week, lives, and possibly thousands of lives, could be lost. And so exactly what you stated does need to happen - “Is abortion ok at 9 months? If not, why not? 8 months? 7 months? 6? 5? 4?” So you pick a number, based in your own reasoning. Let’s say you believe it to be 3 months. After 3 months, abortion should not be allowed. Why? Why not 2 months and 29 days? That one day difference could mean the difference between killing or not killing human beings.
This might all sound pedantic, but once again, my point is, that if human life matters, if human life is the most important thing, which it should be, whatever our morality, then taking chances, making arbitrary time limits, and not always prioritising the possibility that what we’re dealing with might be a human being, is not acceptable. If there is any question, and there is, although you might disagree, that a child at say, 1 month, is a living human being, then we must err on the side of protecting potential life.