If it were a real sport then it might be a bit early but its soccer so it really doesn't count. Its never too early to learn how to grow your hair out, wear bright colorful pre-wrap head bands while running around on a patch of grass. Falling on the ground and crying is probably encouraged by the coaches scouting for the 5-7 year old league.

In my experience with 3 kids, soccer is probably the best sport for them at that age- too many rules to remember to properly hold a game of baseball, football or even basketball. this will boil down to a giant mass with 20 legs all kicking at the ball at the same time, with two kids snoozing in a goal at either end, but it is exercise and a good chance for that player/coach (not relative) relationship to form. I can still remember my middle son scoring 7 goals in one game as a four-year-old, just 6 months after I had broken his leg. Twelve years ago, and still awesome!