In speaking about Harvest Moon, Young stated that the album's theme was, "How do we keep going? How can you keep an old relationship new? How do you make love last? How can you bring the past with you?"
That broader theme fits cleanly and unambiguously with "You And Me." In fact, there may be no one song on the album that better encapsulates it.
The song opens with Young singing:
Open up your eyes
See how life time flies
Open up and let the light back in
From there, the whole song is a call to both remember and renew, to cherish the past while remaining hopeful for the future.
The repeated call to "open up and let the light back in" indicates that, at least temporarily, the light has been lost. Likewise:
Old man sittin' there
Touch of grey, but he don't care
When he hears his children call
subtly suggests that he does indeed care about that grey when the children aren't around and he's left to his silence.
So, the song's not at all naïve or oversimplified, but it is optimistic.