The twins were 10-years-old when the Stark-manufactured bomb decimated their home, killing their parents. Considering Tony moved away from producing weapons in the years surrounding Iron Man, it can be assumed that Wanda and her brother were around 10 in 2008. If they were born somewhere between 1998 and 2000, Wanda would currently be between 18 and 20-years-old in the franchise.
AoU says they were orphaned at 10. Stark didn't make weapons only in 2008. 2008 is just the last year he made weapons before he became Iron Man. They could have been orphaned in 1999. When Olsen would have been 10.