Have been thinking on this for a while, after a friend told me she clearly remembers her 4th birthday party, and another said she remembers her 6th birthday since her dad kept teasing her saying "You're sick?" every time she said she was six.
My earliest memory is a sound - mum shuffling down the long lino passage from her bedroom to the kitchen in the mornings, her slippers sliding along inch by inch. She was paralysed in a car accident - doctors had told her she wouldn't walk again but she proved them wrong! Mum tells me I would have been about 2 at the time. My first birthday party I remember I think I was about 4 too (combined with my brother's birthday party - see Xmas due date thread!).
It made me realise that my children may well remember things that happen, and that we do together, good or bad, from around the ages they are now...which made me aware that I should try to make them positive wherever possible!