I think an 11 or 12 year old is too young for many things. While I know that some 11 and 12 year olds are experimenting with sexual activity, I think it's too young. I think that 11 or 12 year olds are developmentally not ready to understand sex, the implications of having sex or the implications of waiting.
I think that introducing the concept of purity rings, or promises, or pledges to 11 or 12 year olds implies that having sex or not having sex is a decision that they should be making.
My approach with my DS13 has been to provide him with information. This includes factual information such as: this is how a woman becomes pregnant, these are diseases you can contract through sexual activity, here are some ways you can reduce the risks of those things happening. It also includes information about my values and expectations. One of those expectations is that he'll wait until he's older and more mature to make any decisions, including the decision to start having intercourse.
I think that one of the problems with purity rings is that they imply that the choice that preteens and young teens is making is between having sex now and waiting until marriage. Frankly, I don't particularly want my teen doing either of those things. I'd like to see my son delay marriage until he's had a chance to finish college, live independently, get a foothold on a career, and take the time to have a long term relationship with his future wife or husband. I think that if I successfully convinced him to wait for marriage would also create a fair amount of pressure to marry early. But, that doesn't mean that I think he shouldn't delay sex now. I think that there are many benefits to waiting while he's in HS. I think it's better to start as a Jr. rather than as a freshman, and that waiting until college would be ideal.
To me, one of the issues with the purity ring/pledge, and one of the reasons why I think it doesn't work, is that it implies to kids that the decision they should be making is whether or not to have sex before marriage. I think that it's easy for a 14 year old boy facing temptation to think, "well, I'll never make it to marriage, I'm not going to be pure, so why not now?" Instead of setting or breaking a long term promise, I'd rather my kid focused on waiting a little while longer. To me the question that young teen should be asking is "yet or not yet" not "now or never".
