Forgive me if there has been a previous discussion in this vein, but the forum search isn't forthcoming.
I've been trying to tag age categories on young posts that don't have them, but I frequently run into pictures I find ambiguous, especially in the child range. To explain what I mean, let me talk about these three posts currently tagged with child, two of which are examples on the wiki page:
post #1138955 post #4666390 post #5326766
Despite all being tagged the same, these images all look very different. The first has a very stylized, super-deformed look that makes the character look extra-young, to the point they look very similar to posts tagged toddler (such as post #5318726, which, to make things even more confusing, is tagged with both child and toddler). The second I would say looks unambiguously like a prepubescent child, maybe in the age 6-9 range, and is drawn in a relatively realistic style. The character in the third image looks tall and lanky, which I associate with puberty; I'd say they could be either 11, 12, or 13.
Meanwhile, this image is currently tagged with adolescent:
This character to me looks, if anything, younger than the character in the third child image above. They could be 11, 12, 13, maybe 14? It doesn't help that they're sitting down, which makes it harder to judge their size.
Even worse is the boundary between adolescent and adult, because that determines whether a young tag applies at all.
This image is currently tagged adolescent, but I'd say this character could be anywhere from 16 to 20. This is true for a lot of posts drawn in a style that makes characters look more youthful (large eyes and heads, smooth skin, etc.)
Ultimately, I think there are two major problems here:
1. The age ranges for child and adolescent are too broad
2. Defining these categories based on visual age in the first place is misguided, because visual age is extremely subjective
In real life, people can hit puberty and developmental milestones at wildly varying rates. I've met 12-year-olds who are taller than me and college grads who complain about being mistaken for grade schoolers. It's virtually impossible to tell at a glance the difference between a 17-year-old and an 18-year-old. And that's in real life, not in art, which can be drawn in any number of styles that obfuscate common age markers (especially chibi, which can make it unclear whether the character is supposed to be a child or is an adult drawn in a chibi style). Two taggers could look at the same character and, very reasonably, assign them two different ages, entirely due to different experiences with what 3-year-olds, 12-year-olds, and 18-year-olds look like.
We don't need to adopt FurAffinity levels of prescriptiveness on this, but I think it'd be wise to go into more detail in the wiki page descriptions on exactly what visual traits we're using to determine what makes someone a toddler vs. a child vs. an adolescent. I think that, very broadly speaking, those could be delineated as prepubescent / pubescent / postpubescent respectively, but we'd still need to define what those terms mean.
Updated