The Complete Perrage lists Thurstan de Montfort as related to Thurstan de Bastenbourg (d. 1097) and his wife Albereda, the first cousin of Henry, 1st Earl of Warwick. Since Thurstan de Montfort's grandson Thurstan was born about 1184, and his son Henry married his sister-in-law and brother's widow, and therefore likely married quite young, we can assume that Thurstan de Montfort was probably born between 1135 and 1140. This would place him 3 to 4 generations younger than Thrustan de Bastenbourg and Albereda.