I would say once puberty hits, but seeing as how you said you already have a 19 year old I can fully understand why she would want her own room. She's an adult and fully developed and likely doesn't want to live with her little sister who is probably still playing with dolls.
Make Room for Daddy - 1953 Gina from Italy 6-19 was released on: USA: 9 February 1959
Priam, the king of Troy in Greek mythology, is said to have had many children, including 19 sons and several daughters. Among his daughters, the most notable are Cassandra, Hecuba, Polyxena, and Laodice. The exact number of his daughters varies in different accounts, but it is generally acknowledged that he had at least 5 or 6 daughters.
6 daughters
6 to 7 days if your room is at 19 degrees.
Bewitched - 1964 Tabitha's Very Own Samantha 6-19 is rated/received certificates of: Australia:G
204.75 sq ft.
about 1.28 daughters
Bewitched - 1964 Tabitha's Very Own Samantha 6-19 was released on: USA: 29 January 1970
If there are 6 daughters and each daughter has one brother, that means there are 6 brothers. 6 daughters and 6 brothers = 12 children by using addition.
10.6 square meters.
Kraft Television Theatre - 1947 In Albert's Room 6-46 was released on: USA: 19 August 1953
he had 5 daughters and 6 sons