Children and adults can learn piano. 6-7 years old is usually a good stating age for children, although everyone is different. Adults well beyond retirement have also enjoyed learning piano as a new activity.
In the beginning, we start with simple, familiar melodies. Also included is music theory for reading music, and scales and exercises to train the hand movement. After the basics are learned, we can move to other music. Classical music often works best for training at any level, so I encourage studying it even if other music is the emphasis of the lessons.
Within 3-6 weeks, people start to feel like they are gaining momentum. After a few more weeks one can start becoming more selective as to the music they play.