"Millions" is probably rather stretching it, considering that would be substantially older than what is teknik'ly known as "Anatomically Modern Humans" (Homo Sapiens Sapiens is only a few hundred millenia IIRC)...
As Iruru has already demonstrated at least some dragons are certainly quite capable of messing with human bodies magically, but one would assume they can't fiddle with the kind of "deeper" stuff that lifespan extension or whatever would involve. Given what we know of their home plane (read: self-awarely cliché FRPG world) that kind of thing sounds like the bailiwick of the local divinities (who apparently aren't on good terms with the dragons in general), and is probably beyond most of them too.
On a more practical note I rather doubt Kanna would as such particularly care about their different growth rates. Given species differences in aesthetic sensibilities the dragons in general are unlikely to be very concerned with the physical appearance of humans - it's the inside that counts, if anything.
OTOH... in a mere decade Saikawa will be twenty-ish, a quantum leap in terms of mental growth and maturity, while Kanna will remain a juvenile - curious, playful and firmly living in the now. If anyone is going to start occasionally waxing melancholic about their different life-cycles it's likely going to be the former; unlike the adult dragons the latter in universal sub-adult fashion probably can't, and can't be bothered to, consider the long term...
On a more practical note, though, unless she starts adjusting her human form's apparent age she's going to make grown!Saikawa look like a damn pedo. :P