Can I join? Hikari is my favourite girl ever shown in Pocket Monsters :P She's so cute and hot xD
Anyway, in the game I guess she's arround 10, starting as a Pokemon Trainer, but on the other hand you're her at the game so she's in your age :P In the manga she starts off as 10 (as all Pokemon Special characters). On the Anime, hmm Satoshi was 10 when he started (and now I think he's over 14-15, remember the Viridian Episode which he said a year had past? Then he was 11, and in the Pika-short of 3rd movie, he said that it was his anniversary starting as a trainer, then 12, so he might be 13 by the start of Hoenn and 14 by the end). Haruka started at 10 too. I guess he's 4-5 years older than her. Except if she starts on a bigger age (as she's not either challenging the gyms nor being a Coordinator). Well, time will show :P