Why does Mary's face look so young in the Pieta?

Michelangelo's famous Pietà statue in the Vatican, sculpted in 1499, depicts the "Sorrowful Virgin Mary" (Mater dolorosa), holding the body of Christ on her knees after he was unfastened from the Cross and before his burial, Resurrection and Ascension. Mary's face in this statue is particularly youthful. Why is this?

In the early centuries of the Church, there was much debate about the dual nature of Jesus: could he be both Son of Man and Son of God? In 381, the Council of Constantinople proclaimed that "Jesus Christ, only Son of God, true God born of true God, by the Holy Spirit took flesh from the Virgin Mary and became man". The Council of Ephesus in 431 definitively settled the question, giving Mary the title of Theotokos ("who gave birth to God" in Greek). Jesus is both man and God. Mary is the mother of God because she is the mother of Jesus, and Jesus is God.

By giving Mary the features of eternal youth, Michelangelo must have intended to underline her eternal purity, while her son Jesus, having taken on our human nature, was made to appear, in the stripping away of death, as a man like all others.

Translated from www.les-news.fr