Not a parent, but I thought I would chime in as seeing that this topic in general (dating, love, marriage, etc) is something that it very important to me in terms of dignity and respect.
First of all - there is a speaker who speaks on love, marriage, sex, meaning of masculinity, meaning of feminity, etc. He has 3 kids, one 3, one 5 (and one on the way) . They both know basically about how a baby is created and generally what sex is - daddy has the fertilizer, mommy has the seed, etc etc
While this may seem really weird, I think it is absolutly necessary! The sex talk may seem a little awkward, but if you start early, it wont be as bad. Sex and marriage are not something parents should shy away from. Also, dont be afraid to let your kids see you expressing your love for one another. In fact, make sure they do.
If children grow up with a healthy view of what sex really is, it will help them to better understand chastity. It will also help them to better understand purity, and the dignity that they have as human beings.
Chastity shouldnt be thought of as a list of do's and dont's. And I think this is something that is too often overlooked. You are trying to set out a list, what is acceptable when, holding hands, kissing, etc. By doing this, you are promoting an escalation with age. You are also promoting the very fact of doing it. This is contrary to the goal of chastity and purity.
Chastity and purity should have the aim of guarding the heart, soul, and dignity of not only yourself but others. We should not ask ourselves what we can and can't do, or should and shouldn't, but rather should seek a purity of mind and heart and ask what can I do to do that.
Honestly, realistically, and truthfully, dating pre-highschool, and even in most of high school, is superficial and "recreational." It often promotes rebellion against the rules, it promotes promiscuity (the recreation part) in many forms, it promotes objectification, and it promotes a twisted view of dating (in that it is a twisted form of the Truth).
Dating is supposed to be about pursuing a relationship that could lead to marriage. That is the purpose of what dating is. In fact, dating as it is defined today doesnt appeal to me that much. Courting is almost, in my view, much better, much more wholistic, much purer, and much more respectful. In high school, do people date because they can possibly see themselves getting married to this person?
It isnt about age. It is about maturity and faith. I am 19 and a college sophomore, and I have have been in one relationship. That was more than 2 years ago. And I have been so blessed during this time. It is a chance for me to grow in my faith. It is a chance for me to develop and grow, so that if that day does come, I can give myself totally and faithfully and truly to my bride. And I want to know who I am when I am on the altar. I am sorry, but most middle school and high school are huge in the formation of the identity, as well as the person. They shouldnt be focusing on others but on themselves. Also, when entering a relationship, they should not ask "What can I get out of this?" but "What can I give?" It should not be about the self, which too often high school dating is.
IF I get married and have kids, then my kids will know early on about sex and marriage. They will learn as they grow up about dignity and value, and about love. And God willing, from these and other things, they will learn the truth about chastity. And God willing, they will be able to make good educated decisions (should the case arrise that they dont, ....well then good ole dad will help them

). Also, they will be well read on the topic by then. One of the books I have found to be very helpful was "Good News About Sex and Marriage." It is by that speaker I was referring to earlier.
I dont claim to know it all. But what I have said doesnt seem to illogical. And much of it has come from my experiences, and from those around me.