--What kind of bags?
-- What size bags?
-- How do you provide drainage?
-- How many plants per bag?
-- ....
I have not grown potatoes in bags, But I have done it in stack of tires.
Check the stems. Are they roting? If yes then too much water. It is ok for lower leave to yellow and die after about couple of months,just like tomatoes. But drooping? I don't know that one. If you are using plastic bags (especially black one) The roots can get very hot. If potato soil gets warmer than 65-70F, it will not grow tubers. So bags idea might not be very good if it is sitting under the sun and temperatues are high. Try to insulate outsides of the bags (with leaves, hay, straw)to keep the roots cooler. Water them after dark to cook off the roots.

These are so called potato bins from gardener's supply. Like a porous fabric pot. The stems don't seem to be rotting. Just the lower leaves starting to die. I don't know if this is normal or I am doing something wrong. They are in a very sunny spot and the bins/bags are black... Here are some pics. Also, I noticed aphids last week and sprayed them with neem. That problem seems to be better. Could this be aphid damage?
Thanks!
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Here is a link that might be useful: potato bin

Most likely just normal aging.

looks like a typical potted plant problem to me. Po
It could be from heat stress or diseas(less likely). Check the underside of the leaves with magnifying glass for micro organisms.
Another posibility could be nutient deficiency.
I would just pinch off those half-dead leaves. They don't make any contribution anymore and are burden and if diseased, can spread further.
Furtilized a little more. The leave look pale in the photo.

I woulk think that the black color may be responsible for temerature bulid up. Being porous, it also dries up fast.cover around the bags with with hay, leaves etc to lessen the effect. As I mentioned in my first comment, potato root like cool temperature (In your zone potatoes should do well, if planted in the ground. But out in the black porous bag, under sun is another story.