My guess is that he transformed to anything he wants but since he grew to adulthood that's the max he grew to at the time and is most familiar to him. Like, you died as an 18 year old so your memory is up to that time. I haven't even considered your theory about his soul continuing to grow old. That's interesting. But if he's not confined to a particular physical form does that mean he will age physically as well?!! But if we mean aging by dragon years and dragons grow more slowly than humans, usually so in other settings anyways, and he's currently a baby dragon....then idunno. I mean as an 18 y.o. boy I guess he can assume ANY form?!? Cuz technically he's not confined to any physical form. I'm just guessing he's taken on his most familiar form, which is the one right before his death. But you're right, if his soul doesn't age then he'll remain an 18 y.o. forever. And meanwhile his dragon form will keep aging. So, will he be forever 18 y.o. because that's when he died?!? Or will it keep maturing?!? Then the physical form he takes will reflect that?!?
It seems like people don't read the dialogue and just jump to conclusions by scrolling through the illustrations...
they saying "I thought that it was a story about childhood sweethearts not a groomer" like, since chapter one we knew the dragon was a 18 years old human.
Also the dragon obviously sees him as a kid that he wants to protect and, by the looks of it, their romance will probably start developing when the prince becomes a adult