I remember teaching my little brother the alphabet at 2 and helping him with his homework and reading as he got older. Now it's my sister's turn and she's ahead of everyone in her class because she's always reading like me. I also remember being top of mine even though I didn't have an older sibling to help me. I guess I just wanted to learn.
From as little as 1 year old or earlier. I started with my kid at 1 year old. Showing her letters, helping her put them together.
Now at 8, she has been ahead in reading in every single school she's been to. Her grammar and spelling are perfect. She's reading 2 or 3 grade levels ahead.
The earlier, the better.
I honestly don't think there is any "to early of an age". Really you want to expose them to that sort of stuff as soon as you can =)