How old are you?

My sister and I were out in the driveway playing basketball when the little girl next door and another girl went past on their bikes. I asked my sister how old the girl (our neighbour) is and she said "4...5...maybe 3". Then the two little girls came up to our lawn, going "Hello! Hello!". My sister asked "Hey, how many years are you?" The girl said "Four!". Her friend (another neighbour's granddaughter, visiting for Easter) said "I'm this many!" and held up her hand.

Didn't make us much wiser though.
She was wearing a mitten.