All possible truths. Twilight just looked so young to me. It would make more sense if she was closer to 8 at the time of hatching, though, especially considering she got her cutie mark in that flashback.

I do agree that Twilight could have graduated early, but given all the background about Twilight, it wouldn't surprise me if she stayed in Celestia's school as an equivalent to a graduate student, so I'd still peg her as 21–22. With those extra three years knocked off of Spike, though, that would definitely put him closer to Sweetie Belle's age, so that puts her back in the running. More like a schoolyard crush, so it wouldn't really be that boundary-breaking.