I did a project where I taught 20 adults for a semester... and here it is! Yes, it's open access. No paywalls. :)
'Adult Beginner Instrumentalists’ Practice, Self-Regulation, and Self-Efficacy: A Pilot Study' http://redfame.com/journal/index.php/jets/article/view/2978
Most interesting things for me were both the positives and negatives about persistence, goals, and achievement.