There is no ideal age for learning Yoga and there are many styles of Yoga, which focus on the many different aspects within such as Diet, Hygiene, Asanas for strength, Asanas for flexibility, Asanas for better health, Pranayama, Relaxation, Meditation, Enlightenment, and much more.
The fact is; anyone who can breathe can practice Yoga. Older students will be more familiar with their bodies and less prone to injure themselves, due to competitive thinking. Younger students are usually more flexible than older students, but there is much more to Yoga than flexibility.