Just because an argument can be made for other characters in Star Wars being Mary Sue's doesn't mean that Rey is NOT a Mary Sue.
In fact Rey is a classic Mary Sue. Young Anakin is a massive Mary Sue. The two statements are not mutually exclusive.
Therefore the fact Rey is female has zero impact on her being a Mary Sue. Her youth is a factor. That combined with the lack of back story, training montages, build-up and foreshadowing all add up to the result: Mary Sue.