Ginny

[Carol]

Lee wrote:
> And Loona, even though she was in Gini's year, she could have been under 17 or maybe just 17, as we don't know her b-Day.

Carol adds:

Right. Since the battle takes place some time in May, a large number of the students (all except those born later in May or in June, July, or August) will have had their birthdays, so the majority of the sixth years will have turned seventeen (just as most of the sixth years in Harry's year had already had their birthdays by late April and could take the Apparition test. Of the ten students in Slughorn's Potions class, only Harry, Draco, and Ernie were too young.

Just how McGonagall knows that Colin Creevey, who would have been a sixth year and shows up for the battle, is underage, I don't know. (Would she really keep track of every Gryffindor's birthday?) But I think we can assume that she's right simply because JKR has no reason (except possibly difficulty keeping track of her own character's ages since she's bad at math) to have McG be wrong on that point. Flitwick, however, says nothing about Luna's being underage, so I think that we can safely assume she's of age.

As for Ginny, we know from JKR's website that she was born in August, so she's definitely still underage.

Carol, who calculates that about sixteen seventh-years and about thirteen of-age sixth years (excluding the Slytherins, who were dismissed, and the Muggle-borns, who weren't present) *could* have chosen to fight, but we don't know how many actually did