I completely understand that their argument is "God's intention wasn't to give things a false appearance of age, it was simply to make things functional", from which point of view any appearance of age is a spandrel, an unintended, incidental, but unavoidable byproduct. They're addressing the idea that God is trying to fool us. This is how AiG puts it.
God is not a deceiver. He cannot lie. Numbers 23:19 states, “God is not a man, that He should lie, Nor a son of man, that He should repent. Has He said, and will He not do? Or has He spoken, and will He not make it good?” So why would God try and deceive us by creating things with the appearance of “age.” Why would He make the universe look “old” when it is not?
So this is their explanation.
First of all, God obviously created things that were fully functional from the beginning. After all, plants had to be bearing fruit in order to provide sustenance for Adam, Eve, and the animals. He did not just create seeds and wait for them to grow. The created “kinds” had to be capable of reproduction, so they were not created immature. As mentioned, Adam and Eve would have to be able to reproduce in order to fulfill the mandate to be fruitful. This does not imply that these creatures were “created old.” It merely indicates that they were created functional.
But this is a dodge, it's not actually addressing the issue under discussion. It avoids the fact that being created "mature" and "fully functional" still results in an appearance of age. Regardless of whether or not God intended it, that's what we end up with. They are well aware that a "mature" or "fully functional" human still has an appearance of age, so they say this.
The concept of “appearance of age” brings our human experience to bear as we try to determine “how old” something or someone might be. For instance, how would you know what a 35-year-old man looked like if you had no experience watching people grow from birth to age 35?
For example, imagine you were in the Garden of Eden when Adam was created from dust. He appeared there—fully functional. After two hours, if you were asked how old he was, you would say two hours old because you would have no frame of reference watching people grow from infancy to adulthood. So without human experience, it would be impossible to understand the concept of “age.”
But we do have human experience. We also have experience of things like trees and rocks. So what this is actually saying is something like "Even though the universe has a false appearance of age, it doesn't matter because if we didn't know any better we would not even think about the universe having any age, because we would have no point of reference". In other words "If things were totally different to the way they actually are, then it wouldn't matter". But things aren't totally different to the way they are.
This is just gibberish, and it gets worse.
Furthermore, when someone makes the claim that the earth or the universe “looks old,” it is often because they have been indoctrinated to think it looks old. They have accepted man’s ideas about fallible dating methods and approach this issue on that basis. However, the real issue here is to ask what would a “young earth” or a “young universe” look like? After all, if one is sure something looks old, what would it look like before it aged?
This is like saying we've been indoctrinated to think 80 year old people look old, or even that we've been indoctrinated to think babies look young. Finally they get to this.
God has told us the truth in His Word. He originally created many things mature and fully functional. He did not create with the appearance of “age.”
This means, and they're serious about this, nothing in this universe was created with the appearance of age. When the universe was created, nothing in the universe looked like it had been there for any length of time. Adam and Eve looked mature, but they did not look like they were any more than a day old; if you looked at them you would not say "They look like they're 30 years old", you would say "They look like they are no age at all, they have no appearance of age". Rocks looked mature, but they didn't look like they had any age. This is mindless. It's a logical impossibility, as Joshua has incidentally pointed out.
Back to your comment.
Of course it wouldn't necessarily show a specific age, and if it was miraculously created yesterday it wouldn't be a reliable indicator of how old it really was. But this doesn't change the fact that it would still have an appearance of age. And that appearance would be misleading and false.
Well yes, but that's not actually the topic we're addressing here; in the case you mention, the earth has an appearance of extreme age, because it is actually extremely old. And again, what does a "mature" rock look like, as opposed to an "immature" rock?