This is a very helpful thread.
I sense that some of these concerns are a little less hypothetical for those of us who are already living this out, though...

My DD's first "friend" who was more than a friend... was then fourteen, and she was not yet 12. They were classmates, and it was very definitely a mutual crush.
The one thing that my DH and I are trying to do with this particular area of development is to be
non-reactive about it in a general sense.
It's hard when you have a 13yo who has classmates and acquaintances express romantic interest in her... and those people are 15-18yo. KWIM?
There is just no way to make that kind of thing feel 'comfortable' to me as a parent.
We've also found that most of DD's age-mates are incredibly intimidated that she is a high school student while they are in 6th or 7th grade.
The exceptions tend to be kids who are quirky outsiders themselves. DD gets along exceptionally well with kids on the spectrum because they, too, tend to live their lives as outsiders relative to cultural social constructs.
Her current boyfriend is an age-mate with mild Aspie tendencies. They met because he is the
younger sibling of a peer/friend (with pretty severe ASD) that DD met via summer workshops and an extracurricular activity that they are all involved in. I strongly suspect that the only reason that this isn't incredibly awkward and weird is that the two siblings involved
are Aspies. I know that my friends at this age all found it highly creepy if their older/younger brothers were interested in me (happened at least a few times-- once with a brother 3years older than my BFF and I, and once with the younger brother of a good friend who was four years older than myself).
I don't know how long it will last; it seems to be her first "real" relationship of this sort. We're trying to figure out whether or not 13 is "too young" to go to the movies alone, etc. What we are finding is that we don't necessarily have the same opinion on individual items, which is kind of a first for us as parents! For 15 years, we've sort of assumed that we're always on the same page as parents-- to the point that we almost don't need to even discuss things. Now this. LOL.
It's complicated because both kids have had not-terribly-age-appropriate expectations placed upon them since very early childhood (DD because of her disability, and the boyfriend because of his sister's). So neither one of them is "typical" for 13, and this may have been what drew them together in the first place. He knows that DD is very definitely smarter(?) than him, but isn't threatened by it (somewhat to my amazement).
I think that polarbear's comments about it being fairly shallow at this point are absolutely correct. Yes, there are some shared interests, but really, it's the fact that it's mutual and they are both drawn to what they see as "cute" in the opposite sex.
We're pleased that this first boyfriend is NOT 16-17, to be honest. DD is very definitely not prepared for the physical side of that kind of relationship, and some of those 15-17yo classmates and peers
are starting to notice her.
She's a girl who is "one of the guys," only now those guys are starting to take notice of the fact that she's a smart, wise-cracking, funny, blonde...
pretty...
girl. (Cue
'Heyyyyyyyyy... how did SHE get in here?' 
)
Luckily, I was the same kind of girl. Well. I don't think that I necessarily want DD to follow the same kind of path that I followed; I don't think that I
ever went out with anyone my own age. My first spouse was someone that I met when I was fifteen. He was 22.

But what I mean by that is that I am mentally better prepared to 'guide' my DD through what can be some very rocky waters.
I hope, anyway.