sentences:  The youngest British monarch at the start of her reign was Mary, Queen of Scots, who became queen aged 6 days in 1542.The youngest king was Henry VI, who was 8 months and 26 days old at the time of his accession.The youngest queen consort was Isabella of Valois, second wife of Richard II, aged 6 years 11 months and 25 days when she became queen in 1396.