Maybe. Nothing hot about that.
But it's surely something we haven't seen yet, since it's not what we're looking at, and it surely isn't actually just a replica of Allen. Weird.
Maybe she's like Tsunaden in Naruto... AN OLD WOMAN!!
It'll explain why she looks so young, but is the 'eldest child'. =0!
(Wow the page looks empty without all the manga links... I keep accidently scrolling to the bottom of the page >.> )
I thought her true form was more of a location kind of thing - I mean, that her true form can change in appearance but not in what it actually is (Road), so she can look like Allen, or a strangely coloured checkered pattern, but underneath that is Road herself... or whatever.
I don't think she's an old woman, since the Noah clearly haven't been around that long (more than three years, though). Since her word choice can be explained by... er, word choice (technically, Rabi IS still young...), but her actions can't - I'd guess that Road probably hasn't been around much longer than he has. =P She still treats Tiki as an older brother (and father, although jokingly) and Tiki is twenty-six (it says so in his bio), and if she really was that much older then she probably wouldn't.
...As for Road's true form, it's probably just her. ._. I mean, if we were to go into what it actually looks like, and all. It's like getting stabbed through the heart compared to a paper cut, for her... um, other forms? One wounds, the others are minor inconveniences.