As early as possible - within reason. From about 4 years old a child is normally deemed old enough by most teachers to be able to cope with the commitment of learning an instrument.
Learning music requires basic alphabetical and numerical skills, and by age 4 a child should be starting to grasp both of these and be able to use them along with their musical knowledge.
As for adults, it’s never too late to start learning. Many people don’t take up an instrument until they are retired and have more time to commit to their practice. Whenever you decide to start learning, the most important requirement is enthusiasm.
Yes, although it would be best to try and get your own instrument as soon as possible if you decide to continue with your lessons.
There’s nothing wrong, however, with having a few taster lessons before running out and purchasing an instrument.
No, learning an instrument isn’t just about sitting down and playing pieces. Each person learns in their own way, finding certain aspects easier or more difficult than the next person. Therefore, by being in a lesson with another person who is at a similar level to yourself, you can learn from each other as well as from your teacher.
It also helps to build confidence in playing infront of other people, which can be very useful when it comes to exams! Piano is a very solitary instrument, and so the opportunity to learn alongside another should be welcomed.
How long is a piece of string?
The amount of practice very much depends on the individual doing the practice. Ideally, around 2-3 hours a week would be recommended for normal practice, but if you have exams coming up or are working on a particularly difficult piece, it makes sense to put in a little more time and effort.
As far as younger students are concerned, sitting at the piano for 30 minutes each night can be extremely tedious, so encourage them to practice little and often, maybe 10 minutes at a time.
It soon adds up!