Now and Then

[RiverNotch]

I wonder how this got past my radar. Lovely movement, though this needs a bit more grace.
Now and then, I see your face
in a memory, in a picture. Colon instead of period, as these two sentences ring with that sort of continuity.
I wonder how it looks today
under the shade of age.
Once in a while,
since i don't think so much about you
anymore, I agree that this needs to be parenthetical, and because of the unneeded awkwardness of the enjambment, I do think these two lines, however long, should be one straight shot -- if you wanna shorten it, I suggest removing "so much", as the line will say the same thing without it anyway.
I believe time has done away
with its beauty; Once the above is a parenthetical, this semicolon wouldn't need to exist; there wouldn't be any confusion regarding the syntax. That said, these two stanzas read more like one; the added emphasis for or the discontinuity of the coming of age adds nothing to me. Remove the next line's cap, and connect it again only with a comma.

That your eyes don't shine,
and your lips have thinned,
and your skin sags. Also, consider making the rhythm more immediate, more fatal here by removing the commas completely. But only consider -- half of me says there is nothing wrong with the current stateliness.

Other times "Now and then", "Once in a while" , and now this: they all feel like their very existences is supposed to be part of a hidden structure from deep within this poem, but honestly, they are either a bit too cliched or a bit too awkwardly placed to work for me. I tried to play around with other variations (for example, removing "now and then" and displacing it to the two other functions of the poem -- "Now....I believe" and "Then....I imagine"), but so far, none have really sticked out for me, but of course, all his is ultimately up to you. Those modifiers are important, I am very sure, just try and add a bit more dignity to them.
I imagine you dignified,
wearing the crown of experience---The em dash is unnecessary; the break in thought here doesn't actually exist. Just a comma, methinks.
gazing neither behind or ahead, Neither/nor. And possibly remove the comma.
but staring squarely at the moment.
You watch as it passes,
taking with it another I think the enjambment here works as great emphasis of the addressed's age, which works to turn the tale back to the very beginning. Somehow, I do agree that these three lines could be shortened and reworded into two, but as in my thought on removing the commas for your middle (hopefully, soon, the end of your first) stanza, it's not a thought I'm truly sure of.
fine detail of your sunken face... The ellipsis feels hammy, as if it were added here only because this is poetry -- the words do their jobs here well enough, methinks. Colon or em dash instead of it.
Another detail,
like all those gone lost Yes, "gone" is redundant. And this is where all those devices determining time and frequency become important, but again, the devices themselves lack a lot of grace, for me. Nevertheless, great effort over all! Still wondering how I missed this -- or maybe I didn't, and I just forgot.
somewhere in my mind.