There has been research which does suggest that EDs are genetic: (see
http://www.something-fishy.org/isf/genetics.php) but although there might be a genetic link, environmental factors influence the development of eating disorders too. Perhaps in your mother's case, she might have influenced you if she still has traits of her ED behaviours (eg fussiness over calories/ types of food, concern over weight) or perhaps the age is just a coincidence. You did say, after all, that you've had issues with it before now... so that would mean that you would have had an ED at an age before she developed hers?