Theobald I of Lorraine died in May 1220 under mysterious circumstances, and to seal the peace the 19-year-old Theobald IV of Champagne married Lorraine's 16-year-old widow, Gertrude of Dagsburg.  Duke Theobald I's brother Matthew II, who succeeded in Lorraine, made peace and swore fealty to Blanche.  Bishop William of Langres was elected archbishop of Reims in 1219, resulting in his fortuitous departure from the conflict.  Erard renounced any claim on Champagne on 2 November 1221 and Philippa in April 1222.  In return for renouncing his claims, Erard received a substantial cash settlement.  The last rebel baron, Erard II of Chacernay, swore his renewed loyalty to Theobald IV in March 1222.  Two months later in May 1222, Theobald IV reached the age of maturity and began his reign as count, and the triumphant Countess-Regent Blanche withdrew to the Cistercian convent of Argensolles for her retirement.

How many years old was Theobald I of Lorraine's widow when he died?
16