Thanks so much for this important question. It is only in The Urantia Book that we can even come close to an approximation of Joseph's age when Jesus was born.Traditionally, Joseph is pictured as a quite old man, but as we will see, Joseph and Mary were very young when they married and began their family.
122:5.8 "When Joseph was a young man, he was employed by Mary’s father in the work of building an addition to his house, and it was when Mary brought Joseph a cup of water, during a noontime meal, that the courtship of the pair who were destined to become the parents of Jesus really began.
"Joseph and Mary were married, in accordance with Jewish custom, at Mary’s home in the environs of Nazareth when Joseph was twenty-one years old. This marriage concluded a normal courtship of almost two years’ duration. Shortly thereafter they moved into their new home in Nazareth, which had been built by Joseph with the assistance of two of his brothers. The house was located near the foot of the near-by elevated land which so charmingly overlooked the surrounding countryside. In this home, especially prepared, these young and expectant parents had thought to welcome the child of promise, little realizing that this momentous event of a universe was to transpire while they would be absent from home in Bethlehem of Judea."
There's no mention in The Urantia Book of how long Mary and Joseph were married by the time Jesus was born; however, I think we can assume that it was not all that long. Even if they waited a few years, Joseph would still be under 25 years of age, since he was 21 when they married. But in those days, there was no thought of birth control, nor the means to do it reliably, so it seems that reason would conservatively say that Joseph was no older than 25 when Jesus was born.
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Thanks so much for writing. I hope this reply has been helpful