Power Of Not Knowing

I think you're a little off. I think the song as a whole describes a love affair between an older man and a younger girl. I see the first verse as entirely describing the young girl in question. The speaker is talking to a young girl trying to act older than she is–she's embarrassed by how young she seems in comparison to her older peers and is trying to be like them.

In the second stanza, the (presumably) older speaker is trying to impress upon her the gift that is youth. Obviously he feels he's lost some of the redeeming "power" of being unsure and young, perhaps the power of opportunity and being able to more easily steer one's life. He tries to hang on to that in his own life, and searches for someone young to come along.

Then, in the final stanza he expresses regret for perhaps somehow tarnishing this young girl over the course of their relationship. He worries that his presence has stolen her youthful energy and glow through his actions in their relationship and that she can never reclaim it.

I agree with you for the most part, Haruki, although I believe that the guy is the same age but more mature in a way ("our bigger blessing, girl, is being young")--I think he is more self-aware than the girl, who is only eager to grow up, and realizes that their youth is a gift, that being immature is nothing to be ashamed of when you're young, because experience is what furthers the process of learning. His self-awareness, which the girl interprets as him being mature, disarms her and makes her anxious to grow up, but it only managed to make her lose what used to make her special.

I think he is in a relationship with someone who has become jaded, and he blames himself.

Blabbermouth girl embarrassed the guy. She was either too old or too young and couldn't even figure out which one. She had lied about her age, now he cannot trust her, a heart once open will now be closed.