This question is really not about age… the answer depends on your goals and your skill level. For most guitarists you can get most of your guitar education from the younger ages. Guitar knowledge tends to become more solid over the decades because age is not going to be an issue for you. But there are some exceptions…
If you want to try as much as possible to learn as much as possible as fast as possible, guitar teacher and teacher’s assistant are the best solutions. It is only natural to learn the technique faster after you have spent a lot of time playing guitar. If you already have a teacher (especially teacher’s assistant), it is possible to learn a lot of new stuff just from lessons.
You have to have the teacher’s ability and experience in order to learn well too. You do not have a teacher’s ability without having a good teacher – and a good teacher makes it possible to practice a lot. There are some guitar teachers who can teach you and show you how the guitar works.
But the best solution is to listen to the best lesson teachers you can find. You can download free audio CDs, or listen to lessons in your own mp3 player (if you get a CD player installed). Then you just have to go through all the lessons that have been recorded. They will always answer a few questions to get you thinking (like: “What does this sound like?”. So it is a huge advantage to download a lesson! And with good teachers, you will get a lot of lessons…
As an aside, there is no particular age when you should learn guitar. Some young players are good at understanding chord progressions, like they can already make the whole music. For example, this is exactly how me when I was a kid. And I could tell you exactly how I did all those great guitar chords. It was only when I started to learn scales that my hands needed to get used to it. A good student of scales will already have it in them. And it is not an issue to learn a lot of scales with a teacher!
A friend who has no background in any music, only learned to play guitar on the internet. It was all about listening to the teacher. He was not really worried about learning guitar. All he needed was to learn his scales so that he could practice them. Now he plays, a lot, because he is a great student.
I always recommend starting from the basics. Then you will get to experience some really cool guitar
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