my earliest memory is of being in my Church's Nativity play when i was about 3, i was scared and hid behind the donkey and cried! i remember it really vividly.
when i was studying psychology we looked at the way the mind processes memories, and we learned that the way a child's brain works means that they are incapable of holding onto memories in the long term. its not until a child is about 5 or 6 that the things they experience have any chance of becoming real lasting memories. Anything before that age gets lost because their brain just isn't capable of holding onto it. Any "memory" from before that time is a false memory. You may be told about the event by a parent, or see photos or video footage and your mind assimilates that info into a "memory" or at least what you percieve to be a memory.
i don't know if that's 100% true, as i said, my memory of being in that play is very vivid. i found doing psych that almost all studies into the workings of the human mind can be discredited in some way. My husband has no memories from before he was about 6 years old.