Yes Chris, I know what you are describing. I believe I spent most of my childhood feeling that way, especially when I was very young. I remember feeling that time was moving so slowly and I was basically watching myself from a 3rd person viewpoint, just as you described.
I rarely get to feeling like that anymore and I wish i could remember how to get back to it. I don't think it has anything to do with age, but in a way it probably does have to do with the fact that when we get older and get more busy, we tend to spend more time in the moment focusing on work or family, or we are planning ahead, always on the go mentally. When we are younger and not as busy or don't have to be on the go and worry all the time, we have more time to reflect inwardly and sort of keep our mind in a reverie state.
I have two young kids, and being a parent means you don't get to have your mind/brain to yourself anymore. They are constantly saying, "Mom, look" or asking a question, or just thinking out loud talking constantly. I LOVE my kids, but I really do miss having the time to mentally let my mind just wander.
I know at your age you are probably very busy as well, and I don't mean to imply the wrong thing. I'm just saying that from my own standpoint, I don't get much mental down time any more because of having to be at work so early, then pick up the kids, then have to do family stuff or errands. I never get to just zone out anymore. I miss it. I think all this daily responsibility and routine keeps me too grounded. Geez, I sound like I need a vacation.