Interesting discrepancy in that Bailey/Blanchard article. They state that AGP first manifests at or after puberty and tends to be gradual in onset, but may be perceived by the family to be sudden as the young man often hides it (presumably due to embarrassment and shame). I've heard this quite a lot. If a little boy is gender nonconforming and goes on to form a trans identity he's HSTS, whereas a normal boyish boy who comes out as trans after puberty is AGP. The accounts by AGP men of having a fetish for their mother's stockings or whatever from a young age are usually dismissed as retconning and lying, to justify their transness as going back to childhood and therefore not being a sexual fetish.
However, I just heard another one on Twitter, two middle-aged AGP men reminiscing about sneaking to put on their mothers' underwear or their sisters' bathing costume when they were very little.
This is quite common, and Anne Lawrence himself is one of the most vocal about it, talking about covertly wearing his mother's clothes almost as long as he remembers. (Where that train of thought goes is not pleasant, and vividly illustrates just why so many of these men have such an active interest in promoting the concept of the "trans child" and the use of puberty blockers.)
I don't think it's reasonable simply to dismiss all this as lying. (Although I'm not aware of any reports of little boys being caught in their mum's underwear, so I keep an open mind. Maybe they're really good at not getting caught.) I think it's possible these men's fetish may really go back to childhood in some cases, rather than first manifesting at puberty.
The really striking difference between the accounts of the presumed HSTS little boys and these claimed AGP manifestations in childhood is the behaviour. The classic gender nonconforming little boy isn't trying to hide anything. He wants the sparkly bathing costume and the princess costume and the pink shoes. He lets everyone know, and gets upset if his parents try to tell him that these things are for girls, not for boys. He wants to go to school in the Disney princess outfit. These AGP men are describing something quite different. Rather than age-appropriate girly clothes worn in public, these boys know instinctively that their compulsion is shameful and they're embarrassed by it, in a way the HSTS boys are not. They home in on adolescent or adult women's clothing, often tights or underwear. (Richard Wagner was doing it with his sisters' theatrical costumes, though I'm not quite sure just how little he was at that point, and look how that developed.)
It can't be that the AGP boys have been shamed into feeling that their compulsion is shameful; nobody has seen them or commented on it. Whereas people do see the HSTS boys and scold them to get them to stop, but that doesn't faze them. This is all consistent with the observation that boys who come out as AGP trans in later life were normal boyish boys in childhood. The fetish compulsion is always hidden. It's not a part of their normal life of football and climbing trees and building meccano sets.
So I suspect that as more work is done on this it won't so much be the actual age of onset that's the distinguishing factor as the different behaviour. Open insistence on wearing sparkly little-girl princess dresses with no sense of shame, or covert dressing in older women's underwear, with a very pronounced sense of shame. (Although I couldn't discount the lying theory, obviously.) If this is true, what is it that drives that fetish behaviour (or pre-fetish) in such young boys?