Is using aged_up/down and alternate tags together the right tagging?

Motive. ~aged_up ~aged_down solo alternate*

Aged up/down can be understood as alternate age, but it was split into two tags because there are two directions from present. These two tags are different from the other alternate tags and they are mutually exclusive.
I’d write down about some of the reasons why.

Before we discuss, we will not discuss characters that have already appeared in the official works as a different age.

First, because, as abstract as it may seem, there are countless possibilities with aged_up/down. When someone argues that we should use the alternate tags to aged_up/down posts, I'd ask why the current standard should be the standard for that age. The current standard they're arguing for is just one of myriad past/future possibilities.

Secondly, as mentioned above, aged_up/down are an alternate_age, which are out of the standard. It doesn't make logical sense to me to use alternate tags once again for those states that are already out of the standard, and it feels like unnecessary duplication.

Last, this may be a bit off-topic, but is there a guarantee that a character that is smaller than usual is necessarily aged down? It could be that they're bringing back characters from the past, or it could be that they're shorter. Whatever it is, you should choose one or the other, not both.

I'm not sure I'm understanding you correctly, but I think it depends on which alternate_* tags we're talking about.

I think alternate hairstyle should be used without considering whether the character is aged up or aged down. If I see a character without an official younger design and that character has twintails, I would expect the fan-made younger designs to also have twintails. But some artists might get more creative and give her younger self a ponytail instead. I would still expect such posts to be rarer than aged down posts with the canon hairstyle, so tagging those with alternate hairstyle makes sense.

For alternate breast size and alternate height, however, I do think the ages should be considered. If it's a female child being depicted as a teen/adult or vice-versa, it is expected that her breast size and height would change significantly, so there should be no need to add those tags in those cases. But if it's a high-schooler becoming an adult or vice-versa, I wouldn't expect large changes in her physique, so those tags might still be applicable if there are significant differences between the high-schooler and adult versions.

I'm not sure about alternate costume, though. On the one hand, it sounds like we could apply the same logic as alternate hairstyle because many characters have a few regular outfits that they'll be depicted in often. On the other hand, an aged up/aged down character would be much more likely to wear different clothes, especially in the case of school uniforms and outfits that are only appropriate for a certain age range. I think maybe it would be best to only use it if most depictions of the differently-aged character show a common outfit.

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From this, at least alternate breast size with aged up / aged down is correct, but case-by-case you may want to skip it, like when tagging a child (especially non-loli), which you're not supposed to tag breast size on anyway.