Who can practice Yoga?

There are no strict limits as such; however I think we all agree that prevention is better than cure.
Yoga is not generally recommended to menstruating or pregnant women and lactating mothers. Regular breathing exercises during this period of time can help one stay in touch with the practice.

It is very important to remember that since Yoga involves considerable strain on the body parts, extreme care and caution is suggested only under a certified teacher. Loose clothes are obviously recommended to permit freedom in movement.

Yoga is not recommended to pregnant women since it may result in air embolisms during the more complicated exercises. Standing exercises, which include Tadasana and Trikonasana, are recommended to women in their 1st trimester because they promote circulation of blood to the legs and prevent cramps.

Many simple Yoga postures do exist for pregnant women, which help relieve the more annoying symptoms such as morning sickness and constipation. Breathing exercises help in calming down the person and promote smoother and easier delivery.
Whatever Yoga posture you adopt, remember not to exert undue pressure on your abdomen region – if you’re pregnant only of course.

The more strenuous exercises are not recommended to children under 16 years of age because their bodies as a whole have not yet fully developed nor have their glandular systems. They could damage them if they get too eager which is most often the case with children.

Meditation and simple breathing exercises are however, recommended to children because they promote concentration and thinking levels in children.

Yoga is definitely recommended to overly competitive people because they may harm themselves in trying to outdo others. The main purpose of Yoga is to seek peace within yourself and not in competition with others.

References:
http://www.americanYogaassociation.org/general.html
http://www.abc-of-Yoga.com/Yoga-and-health/Yoga-for-pregnancy.asp