Answer based on context:

On 18 October 1216, John contracted dysentery, which would ultimately prove fatal. He died at Newark Castle, Nottinghamshire, and with him the main reason for the fighting. Louis now seemed much more of a threat to baronial interests than John's nine-year-old son, Prince Henry. While Eleanor of Brittany, the grown daughter of John's late elder brother Geoffrey, imprisoned by John since 1202, posed another potential candidate for the crown, the barons passed her over just like their ignorance of her conditions when the Magna Carta was issued, leaving her still in prison. Pierre des Roches, Bishop of Winchester, and a number of barons rushed to have the young Henry be crowned as King of England. London was held by Louis  and therefore could not be used for this coronation so, on 28 October 1216, they brought the boy from the castle at Devizes to Gloucester Abbey in front of a small attendance presided over by a Papal Legate, Guala Bicchieri . They crowned Henry with a band of gold made from a necklace. On 12 November 1216 the Magna Carta was reissued in Henry's name with some of the clauses omitted, including clause 61. The revised charter was sealed by the young king's regent William Marshal.  A great deal of the country was loyal to Prince Louis, with the southwest of England and the Midlands favouring Henry. Marshal was highly respected and he asked the barons not to blame the child Henry for his father's sins. The prevailing sentiment, helped by self-interest, disliked the idea of depriving a boy of his inheritance. Marshal also promised that he and the other regents would rule by Magna Carta. Furthermore, he managed to get support from the Pope, who had already excommunicated Louis.

How many years had Eleanor of Brittany been imprisoned for at the time of John's death?
14