Cub Scouts, as Cindy18 said, start in 1st grade. There are some athletic programs that start at 3 or 4 years old. Little League requires a child to be 5 yrs old by April 1st but there are non-LL t-ball programs that may start younger. Check your parks department and library. There are often playgroups, classes, workshops, etc run through them.
At 4 my son was in our preschool-aged youth group at our church and playing soccer. He went to library programs (some Mom and Me. Some solo) and he attended a few workshops through our parks dept. At that age my DD did our church program and gymnastics in addition to library programs.
at 1:44 PM on Feb. 5, 2010