Marie Stuart became queen when she was 6 days old

Mary I (popularly known in the English-speaking world as Mary, Queen of Scots and, in France, as Marie Stuart) (8 December 1542 – 8 February 1587) was Queen of Scots from 14 December 1542 to 24 July 1567.

She was the only surviving legitimate child of King James V. She was six days old when her father died and made her Queen of Scots. Her mother, Mary of Guise, assumed regency and her daughter was crowned nine months later.

http://en.wikipedia.org/wiki/Mary_I_of_Scotland