DD is 21 months, and she can sing the alphabet song, minus a few letters. She also recognizes a majority of the letters by sight. She learned her letters through flashcards, which we play with every few days.
Around 17-18 months old DD could point to 7-10ish letters when asked, and was signing as many. Around 21 months she could do that with all of them, and at 24 months she could actually say them all. She doesn't know the alphabet song though.
My DD who just turned 3 on Sunday, shows no interest in learning any of that kind of stuff...she wants to be a kid, playing outside, running around, pretending and all sorts of stuff. She loves to read and pretend to read, but if I step in to point letters out and stuff, she gets really made and says "Hey, I was reading that!"
I honestly hadn't even considered teaching the alphabet to my 2 yo. I figure she has her whole life to learn them and we're not in any rush.
She's also in a Montessori school full-time and it isn't something they emphasize at this stage in her development...she does know how to do up buttons by herself though!
My DD will be 4 in January, and she still doesn't know them all by sight. She just recently learned to sing the ABC's (she had to go to speech therapy for being a late talker), so I'm taking it slow with her. She gets frustrated and upset easily, so I'm letting her learn at her own pace.