Definitely edible and even more palatable - in fact I prefer them for making fritters as they have an extra nutty flavour on top of the courgette flavour.
I go for them when they are about 10cm across from plants that usually ripen at 30cm+ (I see no point in growing tasteless monsters, but few orange flesh pumpkins ever become as bland as a marrow). I used to only grow them for the delicious seeds until I discovered the flesh is good grated raw in coleslaw - the only place they can equal a carrot (so I can't speak for pale-flesh types).
Cheers.