When to start out your child out in youth sports and what sport to play are decisions that should take into account a variety of factors.
One of the great things about soccer is that it is a sport that very young kids (age 4 or 5) can start playing because it doesn't require a great deal of skill, the ball is - or should be - small (so that it is easy to kick and won't injure a child if it hits him), and it gets kids exercising and having fun in a low-key environment.
The emphasis at this early age should be on group games, like sharks and minnows, give and go, or monkey in the middle, so that kids can work on building team and social skills.