You do not have to be any age to take lesson. I took my first lesson when I was 13. You have to be 16 to solo and 17 to get your license!
I would estimate to spend $100 an hour, as if you plan on the exact amount (plane instructor) you will always go over, and you will have supplies to stuff, and you will normally fly a bit over an hour!
The most important thing about learning to fly as a kid, is your parents! If you parents support you it will be fine. I remember for the first 6 months my parents paid for all my lessons drove me back and fourth to the airport, ect. As soon as I turned 16 I got a job and started to pay for it all, I got my divers license and drove myself, and my parents (who are great) where totally cool about it! When you get your license cool parents are great to have, as if they do not let you fly and enjoy your license it is not nearly as much fun! I remember telling my mum that me and some friends where off to Vegas for In and Out, and her reply was 'OK enjoy, and Take Care', I felt that was pretty cool, for a mum who son had decided he wanted to go to Vegas for dinner! (I did not tell how much is wwas going to cost though

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Iain