Who is the world's tallest 11 year old?

In 2014, Rumeysa Gelgi of Turkey, age 17, 7 feet 9 inches (213.6 cm), was declared the world's tallest teenager (female) in the world by the Guinness Book of World Records.

The tallest teenage female on record is Canadian Anna Haining Swan (b. 6 August 1846 - d. 5 August 1888), who reached a height of 7 feet 11 inches (241.3 cm).

As for who is the tallest 12 year old, no specific record is kept for 12 year olds, but it is very likely that the 2 record holders above were also the tallest 12 year old girls.