My favourite way to cook them is my attempt at a dish served in a small restaurant near my countryside home. They slice them into segments about 3 - 4 inches long, topped with finely chopped and fried garlic, then steamed.
But the most common way of cooking here is to roll cut them then simply stir fry them. They also go well with eggs or tofu (again stir fried).
The two different types are not a matter of age. They are distinct cultivars. The light ones are Luffa acutangula whereas the darker ones are Luffa aegyptiaca. The light coloured ones are generally my preference. The darker ones have a rougher skin and I tend to use them in soups more. You definitely don't want to be eating old luffa, but then no market should be selling it, unless as a scrubbing sponge / exfoliant.
Sadly, now is not luffa season.
