My first son knew them all by 14 months, but that was because I bought him some flash-cards on a ring, and we went thru them about 6 times a day, every day from the day he was born. For some reason he loved it when I read to him, or did his flash-cards. @ 14 months he could point to any letter (upper case only) and tell you what letter it was. He loved doing it in public, pointing at signs, I think because he liked the reactions that the other people around him would give.
I was a little more busy when my second son came along, so I didn't have as much time to work with him. I'm not actually sure at what age he learned them. He has a severe speech delay, and didn't speak a work until he was 2 1/2. If I asked him to point to a certain letter, he could do it though, so I think he knew them. He is 3 now, he knows all his letters and what sounds they make, and we're slowly making progress with his speech.
at 10:28 AM on May. 6, 2009