It's definitely only a decision for you to make but I was diagnosed in 2003 at age 35 and had a single mastectomy, only to have a new cancer diagnosed in the other side in 2010 (resulting in another mastectomy and this time chemo too). I wish that I had made the decision to have them both off at the same time......it's all good to look at it retrospectively though coz I wasn't actually ready to have a perfectly healthy breast removed at the time that I had the first one done!!
Good luck with the decision,
Louise