As a recent position statement by the American Medical Society for Sports Medicine notes, coaches and parents "often lack knowledge about normal development and signs of readiness for certain tasks, both physically and pyschologically."

I agree with the AMSSM that there is "no simple way to determine if a child is ready for a particular sport."

When to Start

Consider your child's physical growth and developmental readiness:

• Basic motor skills: A child needs to have mastered basic skills (running, throwing, balance and ability to track objects and judge speeds) before she begins sports. Unfortunately, according to one study, about half start without those skills. Your child is mature enough to play a team sport if she has a long enough attention span and enough self-discipline to learn from group instruction. remember: Playing sports won't speed up the maturation process.
• No set age.  Chronological age is not a good indicator on which to base a child's readiness for a particular sport or organized sports in general. As with other child developmental milestones, motor skills develop at different rates among individuals. While there is therefore no set chronological age that will guarantee mastery of a certain skill, most children aren't ready to participate in organized sports until they are four or five (and even then, only on a limited basis). Experts recommend against team sports before age six and that contact sports wait until middle school. 
• Cognitive development must occur before the young athlete can participate in most organized sports. In early childhood, the young athlete may not understand the need to stay in position or to be able to remember instructions. To enjoy a sport, a child needs to understand the fundamental rules and strategy of the sport. 

Ask yourself if you are ready

Watching a young child play sports will trigger your natural instinct to protect and nurture. If you think you would probably rush on to the field if your child was injured in a game, you may not be ready to handle the stress of your child's participation.

You know your child better than anyone else. It's okay to go with what your instincts tell you are in the best interests of your child. It's the kind of thing mothers have been doing for eons.

Picking a Sport

1. Consider your child's interests, temperament and level of commitment and motivation. Ask him what sport he wants to play. Look for clues about the sport he might like in how he spends his free time (throwing=baseball; kicking a ball=soccer, hanging from a jungle gym or tree=gymnastics, playing alone=individual sport, playing with friends=team sport). Kids who are prone to anger and are hard on themselves when they fail may be better suited for team sports. Make sure the level of commitment is there if your child expresses an interest in a sport like ice hockey, ice skating, gymnastics or swimming that have long seasons or are practiced almost year round, even at an early age.
2. Think outside the box: There are many, many different sports your child could play besides soccer, baseball and basketball. Consider family-focused activities, like bike riding, hiking, kayaking, swimming or golf that can help a child stay fit while having fun and have the added benefit of giving the whole family an activity to enjoy together. Don't automatically say no to a boy who expresses an interest in dance or yoga, or a girl who wants to play football or wrestle.
3. Consider your child's size. If your child is small for his age, consider sports that emphasize quickness, agility and balance, rather than strength, like gymnastics and dancing, or endurance sports, like distance running and swimming.
4. Check out the program. Look for one that emphasizes having fun, skill development, equal playing time and fair play, and that keeps winning, losing and competition in perspective. Talk to the coaches to make sure their philosophy is the same is yours. Attend a game. You should hear kids having fun, not yelling by coaches and parents.
5. Consider the cost. The cost of some sports (hockey, gymnastics, figure skating) can end up being a budget buster for many families, so it is important to consider the true coast of sports participation before registering your child.
6. Set realistic expectations. Your goals for your child should be to stay or become physically fit, have fun, make new friends, develop skills and start down the road to a lifetime of physical activity. You role is to love your children, protect them from harm and help them realize their dreams, not to realize your dreams.  

• Ages 2 to 5 (early childhood): because children this age have limited fundamental skills and poor balance, appropriate activities include running, swimming, tumbling, throwing, and catching.
• Ages 6 to 9 (middle childhood): posture and balance are becoming more automatic, and reaction times have improved, so activitiescan include running, swimming, skiing, entry-level soccer, baseball, tennis, gymnastics, and martial arts.
• Ages 10 to 12: most can master complex motor skills but may have a temporary decline in balance skills during the pubertal growth spurt; entry level complex skill sports are appropriate, including football (although an increasing number of experts, as well as the American Academy of Pediatrics, are recommending against footbal at this age, some arguing against football before age 14), basketball, hockey (although, of course, many kids begin organized hockey much earlier), and volleyball. 

Sources: 

De Lench B, Home Team Advantage: The Critical Role of Mothers in Youth Sports (New York: Harper Collins 2006)

DiFiori JP, Benjamin HJ, Brenner J, Gregory A, Jayanthi N, Landry GL, Luke A. Overuse Injuries and Burnout in Youth Sports: A Position Statement from the American Medical Society for Sports Medicine.  Clin J Sports Med. 2014;24(1):3-20.

Brooke de Lench is Executive Director of the non-profit MomsTEAM Institute,  Founder and Publisher of MomsTeam.com, and Producer/Director/Creator of the PBS documentary, "The Smartest Team: Making High School Football Safer." She can be reached by email at delench@MomsTeam.com, and you can follow her on Twitter @brookedelench.