This is just my two cents:
For algebra, DC15 took it early and still struggles with some of the abstract thinking. DC20 asked me a lot of questions about that earlier (not his area of strength or interest) and sort of figured it out. I think it really depends on the child.
As for "helping," my father helped me learn a lot of math before I started school ("Daddy, how to I calculate what shake is most likely in Yahtzee?" "If dice had 8 sides, would it be 1/8?"), and I loved math and anticipated the day that I would start school. However, my teachers believed in optimum times to learn things, and I soon started to hate mathematics. I didn't start to enjoy it again until I worked with it on my own after undergraduate studies. I don't think that being early would have been damaging or too difficult; however, learning it too late led me to believe that it wasn't interesting and that I wasn't too good at it...
