How old was Joseph when he became engaged to Mary mother of Christ? What happened to him?

We don’t know how old he was at his engagement, although he was old enough to be responsible to a Roman census and to be required to leave Nazareth to attend his family’s home town, Bethlehem.

The last we see of Joseph is in Luke 2 when Jesus was 12 years old.

Luke 2:41-42  Now his parents went to Jerusalem every year at the Feast of the Passover.  (42)  And when he was twelve years old, they went up according to custom.

Early in Jesus’s ministry Mary and Jesus’s brothers are referred to, but without any mention of Joseph, suggesting that Mary was a widow.

John 2:12 After this he went down to Capernaum, with his mother and his brothers and his disciples, and they stayed there for a few days.

When Jesus was crucified he appointed the disciple John to take her to his home, suggesting that Joseph was not alive.

John 19:26-27  When Jesus saw his mother and the disciple whom he loved standing nearby, he said to his mother, “Woman, behold, your son!”  (27)  Then he said to the disciple, “Behold, your mother!” And from that hour the disciple took her to his own home.

So, although the Bible doesn’t tell us directly about what happened to Joseph, it seems likely that he died relatively early on in Jesus’ life.