Basically, much of 1.0's early storyline was actually bouncing in and out of the Echo, specifically others' memories of the year 1562 6AE when the Archons (namely, Yda, Papalymo, Thancred and Y'shtola) first arrived in Eorzea under directions from Louisoix. Back in 1.0, the Echo was rendered differently to how it is here in ARR - in 1.0 it was virtually indistinguishable from reality (you can still see this if you view the 1.0 opening cinema movie which is found easily enough on Youtube), and the only way you initially could tell the difference was an Echo scene begun with a subtle woosh effect and a soft gong sound. Other than that, you had absolutely no idea before Minfillia taught you how to harness the Echo at will, when it would literally ask you to use it and so there was no confusion.
Also Thancred was definetely an adult (or at least in his late teens) when he first arrived in Ul'dah - I still have screenshots of the scene in question of my old 1.0 alt that started in Ul'dah and he was clearly physically (and acted like) an adult:
I'm not sure about Thancred's exact age though as it was more Gridania's 1.0 story I was taking notice of, so Professor Moose or one of the other 1.0 loremasters would be better to answer this question as they're much more knowledgable on the subject than me.