Now and Then

[Leanne]

Hi ellz -- with a slightly lighter touch you will have a poignant, musing poem.  

Now and then, I see your face -- I am ambivalent about "now and then" -- it's quite generic, but that might not be a bad thing in this case, as if it's just said casually in passing.  Still, I can't help thinking that the opening line would work just as well (maybe better) without it.  Depends what you're going for.
in a memory, in a picture.
I wonder how it looks today
under the shade of age. -- if you use "beneath" instead of "under", you capture both the long "e" sounds that match the long "o" and "a" around it and it also fits better for meter when read aloud
Once in a while,
since i don't think so much about you
anymore, -- you could consider putting this part in parentheses, or at least between em-dashes to make it an aside 
I believe time has done away
with its beauty; -- your phrasing is unclear here.  Although I know (because I'm paying attention) that the beauty you refer to is of the face, the way you've written it makes it seem as if it's time's beauty.

That your eyes don't shine, -- consider removing the capital letter at the start of this line, as it is enjambed between strophes and a cap breaks the flow
and your lips have thinned,
and your skin sags.

Other times 
I imagine you dignified,
wearing the crown of experience---
gazing neither behind or ahead, -- neither should be partnered with nor, not or
but staring squarely at the moment. -- nice assonance
You watch as it passes,
taking with it another 
fine detail of your sunken face... -- these three lines could be shortened into two -- the "passes" bit isn't really required, but rather you could consider "you watch as it takes..." -- also, the ellipsis here seems fairly cheesy.  You could just put in a strophe break instead.

Another detail,
like all those gone lost -- "gone" is redundant
somewhere in my mind.