Not very old, too young to be on its own! The ears are standing up well, I would say about 4-5 weeks. it should be at the point of lapping up some KMR, kitten replacement milk, (NO cow's milk!) or goat's milk. (at Walmart) Maybe even wet cat food, thinned down with milk or water, Poor little thing, thank goodness it has you! Bless you!
Agree with the above guesses. You could also weigh the little one. Its said they gain a pound a month pretty steadily through the first 6 months. Unreliable, but its interesting to know regardless.