I don't think I ever pictured Anakin as a teenager or even a very young adult. I always pictured him as 35 yo dude, at least, maybe even 40, who was trained by Obi-Wan Kenobi and always was a friend to him for years and years until something happens and THAT's when the movie start...

But I guess I always was wrong because Obi-Wan says that he hoped he could have trained Anakin as well as Yoda and he failed. So I guess Anakin always was quite young dude in Lucas's mind when he turns to the dark side. But it's not how I pictured him for some reasons.