The word "Yoga" means differently to each person.
If it is just physical exercise, then it is better to find a genuine teacher who knows ATLEAST basic physiology, other wise, teaching something like "shirsha Asana" (upside down on bare head) can get very good appreciation, but may PERMANENTLY damage minute nerves in brain etc, and make that child dull for ever !
There is no lower or even upper age limit for any Yoga Aasana. Just that the prescribed exercises (or Aasana) should suit that individual's body condition at that time ! For example, a power lung exercise like Kapal Bhaati could trigger even heart problems to a weak-heart adult. Only a capable & Sincere guide (may be a teacher, master, guru.. whatever) can give correct & suitable prescriptions.
Any posture is an Aasana.
Those Aasanas which take you to Yoga are Yoga Aasanas.
Yoga in that sense is experiencing our one-ness with the Universal One.