In our office we installed a large 6′ x 6′ green wall full of Pothos. It’s been almost three years and we’re starting to notice they are much darker with many long strands. Are these plants getting old? Should we replace them all?
Pothos will sometimes put out darker leaves depending on the amount of light they receive. If the plants you started with were lighter or "marbled" they were probably grown in optimum light conditions - indirect, bright light. If your setting is darker, the plant will use it's energy to grow, it can only generate energy from the green portion of it's leaves if it is not getting enough light, it will compensate by putting out mostly green, darker leaves.
As to the longer strands, this is a normal growth pattern of the Pothos. If you want the plant to produce more from the base you need to trim off the longer strands.