Well, it ain't in the bible... merely, the testimony of Church tradition for at least 1800 years.
Several infancy narratives, from which Catholics also establish the identities and traditions surrounding St. Ann and St. Joaquin, describe it. These narratives date back to very early Christian history. There are some other hints, however:
The narratives tell of how St. Ann was barren at an old age, and swore to God that she would give her son to the Temple if she were to become pregnant. When she became pregnant, she contracted to give her son up. Oops. It was a girl.
The Temple leaders took the child, believing somehow it was the will of God. And the child took an oath of perpetual virginity. But when the girl approached puberty, she had to married off. They assembled the young men, and released a dove, and planned to name as her husband the man the dove flew to. But the dove flew away and landed on Joseph's staff, which instantly bloomed, confirming God's will, even though he was an old man. And that's why an old man was married to a young girl.
Now, if you were going to try to make up an explanation for some of the stranger elements of Mary's backstory, you could come up with alternate explanations, no doubt, but the story does explain:
Why Mary, a near child, marries an old man; why a married woman with child is recognized by the prophets of the temple as being a demonstration of a prophecy that a virgin would give birth; how Elizabeth can likewise make such a realization (note they say they have SEEN the fulfillment, not that they SENSE the impending fulfillment); What happened to Joseph; How the child of a WOMAN from a rabbinical family and a carpenter ends up so knowledgeable about scripture*.
The Greeks also use this story to explain how Mary had no other children (cf, eg. Jn. 19:26), yet Jesus had "brothers": Joseph was an aged widower with grown children when he married Mary. Certainly, Mary of Clopas/Alphaeus had children with the same names as the "brothers" of Jesus, who were among Jesus disciples, including James the elder, who appears to be the one who calls himself "the brother of Jesus."
[*Acknowledgedly, many Christians write this off to psychic forecasting. I don't believe Jesus used "magic," or else he "could" have healed the residents of Nazareth; it is faith which healed the sick, not the use of magic-like powers, hence he refers to what "The Son of Man" can do. I believe prophecy stems from an understanding of the ways of God, Man, and Natural Law through an indwelling of the Holy Spirit, thus a prophet anticipates the future, but doesn't psychically divine it. God the Son COULD do anything, but he CHOSE to be fully human. Hence, in the Desert, he has to DECIDE how his ministry will play out, and thus, he faces temptation. There is no temptation if he is consciously aware of all future things without use of reason.]
Thanks.