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Id say around 8-10

Bioshock 2 is said to occur somewhere around 10 years after Bioshock, and all of the Little Sisters were estimated to be in the age range of 6 - 8, so I would have to estimate between 16 and 18. The Big sister are post adolescent little sisters ( in other words Teenagers) so they would be from 15-18. Also they are big sisters that are of course 16-18 but their is also 20 or so year olds too!