We are going to take our son out for a projected five-year cruise in 2009, when he turns eight. We have averaged out the various aspects of the wisdom of this, and we figure that eight-year-olds have just enough self-preservation instincts to not require constant care, while 13 is about as old as he will get before he a) starts going all hormonal and sulky, or b) starts showing enough maturity to earn a vote on whether or not he wants to continue living on a boat.
We expect by 10 he will be able to stand day watches. At four, he could handle light-air tillering on a 33 footer without rounding up, and I think he is beginning to intuit sail handling. We want to put him in "junior sail" courses as early as possible, which would be next year in Optimist prams. This year is swim lessons in Lake Ontario and COB exercises on the new, higher freeboard boat.
I would encourage you to give your daughter as much responsibility as she can handle in the running of the boat. This would include look-out, checking for water flow when motoring, log keeping, and identifying sail sets, weather formations, radio monitoring and stuff that may interest her, such as fishing, birding or working the windlass (gratifyingly loud and filthy work).
Certainly, if you make a point of living on and working from the boat in all weathers (and make for your daughter a private space of her own), then you will inculcate a sense in all of you that boat life is "normal" and in fact desirable.
Needless to say, taking a youngster is likely to be equal parts hard work, emotional management and organization discipline (they have to do two to three hours a day of concentrated lessons, after all). But the rewards, we feel, will be many, and we believe, perhaps with equal parts pessimism and realism, that the next ten years might be the last opportunity to cruise the world relatively freely, and to see certain places and natural wonders before they become off-limits, extinct or eroded to sand.
Anyway, start her young!