Beware: this post may contain too much information!

I started masturbating with my mother's vibrator as soon as I found it and realized what it was for. Around age 13. Before that, I used some... well let's just say random household objects. Many kids know what's up; and many are like me, relentless snoops, and/or fairly innovative. I figured out where my dad hid his girly mags when I was about 10. Same with mom's "special undies". I would try them on and look at the magazines while they were at work and I was supposed to be doing homework. I never thought about a vibrator, and it was in a different drawer than the undies. One day I found it while looking for something else... regular socks, I think.
In my family we didn't (and in fact, still don't) talk about sex, sexuality, our bodies, etc. In the event that I were to end up with teenagers in my house, I'd likely be way more up front, and sex-positive (of course with the parents' permission). If a teenage girl/daughter-figure came to me asking about such things, I'd educate her to the best of my abilities and to the extent that her parent(s) were comfortable and if she asked me to help her obtain a vibrator or dildo, I would (with folks' permission) help her research the best one for her and pick it up for her, if for no other reason than to keep her from using mine! I feel like going to a grown up --whether it be a parent or a trusted family friend and asking to have a frank discussion about sexuality is a pretty good sign that the kid is ready to engage in such a discussion, and truthful, accurate information about the pros/cons of whatever is discussed should be made available to the person on an age-appropriate basis.
Not that I don't love who I am now, but there would have been a lot less trial-and-error in getting here if I'd been raised in a different environment.

I agree with oohlookasquirrel. It's not a matter of "letting" kids do anything. They'll do what they're gonna do. It ought to be about providing kids with relevant information as they grow up, so that they know what's real and what's not. I would also say that if you find your vibrator, electric toothbrush or electric razor eating through batteries at an alarming rate, and have a teenage daughter (or maybe even son), it might be an appropriate time to bring up or revisit the subject of masturbation.

I'd give them a small, cute, non intimidating toy (think hello kitty "shoulder massager") at a very young age, without precising anything about it. When they'll be ready, they'll have it on hand. I'll never leave my child toyless as I was. It can lead to injury (improper use of household items) and distress...

Also I prefer them having this handy than getting pregnant at a young age because they had too much tension built up.

The first time I masturbated was 14. I think I would probably had an easier time if I had a toy. Honestly, whenever they are ready... it's a vibrator, how much harm can it do?

I think it's up to the child to decide when they are ready.

As long as they're safe and informed, I would pretend not to know, but be open in case they needed me.

Well, if I had a daughter and she asked for one knowing the purpose of one, I would assume she needed one and get her one. It would be awkward but I can think of worse things a kid could ask for.

Whenever she expressed interest, we'd have the discussion.

I would be totally comfortable with helping a daughter of mine (if/when I have one) get a vibrator when they expressed interest. I plan on letting my kids have access to plenty of educational/informativ e material about sex and am perfectly happy to answer their questions. There's nothing shameful or weird about masturbating or vibrators.