St.Joseph and St.John numerous portrait depicting and older man is good way for the artist to convey an image of a chaste man. We know that a young man struggles to control the urges of flesh which is associated to concupiscence.Even St.Paul who experienced mystical conversion still had to struggle with the law of the flesh.
Romans 7:15-24 New International Version (NIV)
15 I do not understand what I do. For what I want to do I do not do, but what I hate I do. 16 And if I do what I do not want to do, I agree that the law is good. 17 As it is, it is no longer I myself who do it, but it is sin living in me. 18 For I know that good itself does not dwell in me, that is, in my sinful nature.[a] For I have the desire to do what is good, but I cannot carry it out. 19 For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. 20 Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.
21 So I find this law at work: Although I want to do good, evil is right there with me. 22 For in my inner being I delight in God’s law; 23 but I see another law at work in me, waging war against the law of my mind and making me a prisoner of the law of sin at work within me. 24 What a wretched man I am! Who will rescue me from this body that is subject to death?
Artist depicting a young man would convey doubts, if St.Joseph can be able to hold on his Church given title invoking his powerful intercession on the following litanies;
LITANY OF SAINT JOSEPH
Illustrious Scion of David,
Light of Patriarchs,
Spouse of the Mother of God,
Chaste guardian of the Virgin,
Foster-father of the Son of God,
Watchful defender of Christ,
Head of the Holy Family,
Joseph most just,
Joseph most chaste,
Joseph most prudent,
Joseph most valiant,
Joseph most obedient,
Joseph most faithful,
Mirror of patience,
Lover of poverty,
Model of workmen,
Example to parents,
Guardian of virgins,
Pillar of families,
Solace of the afflicted,
Hope of the sick,
Patron of the dying,
Terror of demons,
Protector of the Church, pray for us.
Prayers and Litany of Saint Joseph
From the Litany of St.Joseph which the Church acknowledged, it depicted a young and strong worker and defender. It is nowhere described St.Joseph as an old man.Let's see how Blessed Catherine Emmerich describe the appearance of St.Joseph before marrying the Blessed Virgin Mary.
Then I saw that messengers were sent throughout the land and all unmarried men of the line of David summoned to the Temple. When these were assembled in large numbers at the Temple in festal garments, the Blessed Virgin was presented to them. Among them I saw a very devout youth from the region of Bethlehem; he had always prayed with great fervor for the fulfillment of the Promise, and I discerned in his heart an ardent longing to become Mary’s husband. She, however, withdrew again into her cell in tears, unable to bear the thought that she should not remain a virgin.
MARRIAGE OF THE BLESSED VIRGIN TO JOSEPH (Catherine Emmerich)
Based on the sketch drawing of the vision of Blessed Catherine Emmerich, if we based it on the caricature of the face and try to match it with available painting the closest I found of a young image of St.Joseph is this one;
The biblical account of St.Joseph in the "Flight into Egypt" following the angel command for him to accompanied Jesus & Mary to travel by foot from Jerusalem to Egypt, an old man could not make it physically. Not to mentioned they will return to Jerusalem after King Herod death by foot again. Also, carpentry works requires physical strength which St.Joseph teaches for 18 long years Jesus hidden life in Galilee.
In closing, St.Joseph depicted by so many artist as an old does not represent his true physical appearance but more of an artist way of presenting a just man living a chaste life. An old man depicted and acknowledge by the world as already passed or no longer interested in the affairs of the flesh.