I've read all of the responses, and I'm sorry that you received some not so nice ones.
It's hard to say what the right decision is. You know your daughter best, we don't. Maybe there are some 13 year olds out there that shouldn't be dating b/c they can't handle it, and maybe there are others out there that can start dating. Who is to say if a person is ready to date or not when they don't personally know the people at hand?
Now to the part about dating a guy who is 17. Yes, some people might find that weird. But when I was 16 I dated a guy who was 13. Now, at the time I didn't know how old he really was. I went on a double date/blind date with my friend and her boyfriend. But at the time I really hit it off with my date. When I realized his age I was shocked because I assumed he was my age, he certainly looked my age. Did it last? No, of course not. it only lasted 3 months, but there were problems in that relationship that had nothing to do with age and I won't get into that.
Now, at least you know how old the boy is, I kept it from my parents because I didn't want to hear what they'd have to say about it. But looking back I think my mom knew.
I have always been close to my mom, and I could always talk to her about anything. My dad on the other hand.....hmm... Yeah we didn't get along very well. I think once I started college and moved out on my own was when my dad and I started to bond, but not so much with growing up.
Now, back to the boy. How does she know him? Where'd they meet? You know him, but have you met his parents? What do his parents think about him dating someone almost 4 years apart in age? Do they have a problem with it? Those are questions to ask.
But also remember if you tell your daughter no, and she really wants to do something, she'll go behind your back and do it. I know people said all boys think about is sex, and I won't deny that or say it's not true because it is. I work with high school kids, I'm not blind. But just cause he's a guy and sex is on the brain, doesn't mean he will pressure your child. And again if he doesn't pressure her, who is to say she doesn't feel that pressure in school? Or elsewhere? The world is changing and things are different now than when we were kids. You can only protect your kids so far.
I think you and your husband should talk. I think he isn't fair about saying what she can and can not do, especially since you two have already agreed to let her date. Why change things now? That's not fair to your daughter and she won't understand it.
Sorry for writing a book.