Kittens won't eat

My kittens are 4 1/2 weeks old now and I cannot get them to eat any food. I've tried three different types of wet food, made into a mush by adding a little water, but they don't seem the least bit curious.
They seem very well developed, and look like they should be starting to eat, but its a no-go.
Are they still too young? At what age to they start eating food?

They should start to become curious about the food at this age. I usually begin providing wet food (mixed with some kitten glop if they're orphaned) around 4-5 weeks.

Sometimes, with a bowl of food wet mushy food, I will touch the kitten's face to the food. Just enough so their nose/lips graze the surface of the food....They get it on their face, and lick it off to clean themselves, and you can literally SEE the lightbulb go off over their little heads....hey! this is FOOD! This has worked for me MANY times in the past.