I think it's something that has to be kept secret. For her, who can fight people on a mental level without raising a finger and jump through space without an arc, she might as well use her "real" body. But she doesn't. Even if Noah age slower, They still age, so the Earl aside, I'd expect Road's real body to have gained at least a couple of years. (It is possible that the aging slows/stops as soon as the Noah memory takes over. So the Earl is most likely the only one who has halted aging completely, once Mana was overridden by the Noah memory). Anyway, since Road does not use her body at all, I honestly think that she's either:
a) physically disabled due to injury/illness
b) disabled due to age (her true body is like a 100+ year old granny, and she's reliving her life/childhood vicariously through her dream persona)
c) is ill and in a coma herself (and therefore can only communicate through her dream persona
d) she is, in-fact, a doll, meaning that there was never any human host for this Noah memory