We know from context that Eleven is approximately twelve years old, based on how long she was missing.
"Twelve years? Twelve years she's been looking for her. And then she [Eleven]
shows up at Benny's five nights ago, which means we've got a chance."
Chapter 6 - The Monster
Interestingly, in the original draft script she was explicitly stated to be ten years of age.
A YOUNG GIRL, 10, emerges out of the woods.
Kali's age isn't stated, but we see her in a UK primary school uniform in one of the newspaper articles about her disappearance. That means that she was at least five and no more than eleven years of age when she was taken, presumably to act as a control subject.
Since we know how long the experiments lasted (e.g. 12 years) we can place Kali's age at about 17 to 23 years of age. Obviously this could be subject to change if we find out that she was abducted prior to Eleven's abduction.