I just don't think Eysseric physically developed much more after 16 or 17. Still, he's improving and there's plenty of time.
Pless was brilliant in juniors, as you said final at the USO but also final at Wimby and won the AO over Youzhny.
What's obvious is that the Australian juniors doesn't count for a ton. Plenty of the top juniors don't play it and you have some winners that are not that talented. Sidorenko is one, Eysseric is another, and a third Frenchmen that won it is Clement Morel(though I think he ended up playing for Monaco)
Also you can throw Klein into this category.
Klein, yup, won AO juniors in 2008. But he's not that talented so I'm not surprised he has struggled, and of course most know him to be volatile.
Eysseric is a scrawny guy physically for sure, guess he simply develops at a physically slower pace than other players. I've never been impressed by what he's brought to the court, however. I think he was totally overrated from the beginning.
Morel did play for Monaco. I saw him play too a few years back, and saw nothing in terms of ability.
You're right about AO and I guess ultimately, it comes down to some guys developing mentally and physically and peaking at a little bit later age and developing throughout their years, instead of peaking at 16-19 and then not developing further as players.