Now and Then

[NakedBear]

Now and then, I see your face
in a memory, in a picture.
I wonder how it looks today
under the shade of age. Beneath instead of under
Once in a while, possible remove this line - it feels clunky to me and removing to change the next line smooths this over while giving new opportunities

you were talking faces in the first lines, now you talk of the person. I'd prefer a line break and a shift of object.
since i don't think so much about you I don't think much of you
anymore,
I believe time has done away but believe that time has done away (I don't like the 'I' here, but 'but' links better with what came above. I do like 'has done away'
with its beauty;  your instead of its

That your eyes don't shine,
and your lips have thinned,
and your skin sags. to strengthen the parallels to the previous two lines you can add change a bit to 'has sagged'. Unless the break in parallel is to bring emphasis. If it is for emphasis, I think it is weak. Plus I'd wonder why you emphasis that. but never mind, I'm rambling.

Other times When I do
I imagine you dignified, I imagine you
dignified
wearing the crown of experience---
gazing neither behind or ahead, nor instead of or
but staring squarely at the moment. how does one stare at a moment? I'm curious. Maybe a different mode of experiencing a moment would work. I don't know what just now.
You watch as it passes,
taking with it another 
fine detail of your sunken face... is her face sunken? that description seems incongruous with dignity. Sure, you don't need to be pretty to be dignified, but the word itself has no dignity.
Another detail,
like all those gone lost is lost to you
somewhere in my mind. as many are
to my mind.

Anyway, I do like the sentiment. I'm a sucker for sad romantic feels and all that. I won't be hurt if you ignore my suggestions - they are only suggestions.