The year of Alfonso's birth is not recorded in the medieval documentation. According to one of the authors of the "Anonymous Chronicle of Sahagún", who met the monarch and was present at his death, he died at age 62 after reigning 44 years. This indicates that he was born in the second half of 1047 or in the first half of 1048. Pelagius of Oviedo wrote that Alfonso was 79 when he died, but that would place his birth around 1030, before his parents' marriage.