Muslims are quick to attack to the age of Mary when she got married to Joseph, when we criticise Muhammad for marrying Aisha at the age of 6 (or 7), and consummating the marriage three years later (when she was 9):

Sahih al-Bukhari 3894,
Narrated Aishah: My marriage (wedding) contract with the Prophet was written when I was a girl of six (years). We came to Al-Madina and we dismounted at the place of Bani Al-Harith bin Khazraj. Then I got ill and my hair fell down. Later on, my hair grew (again) and my mother, Umm Ruman, came to me while I was playing in a swing with some of my girl friends. She called me, and I went to her, not knowing what she wanted to do to me. She caught me by the hand and made me stand at the door of the house. I was breathless then, and when my breathing became normal, she took some water and rubbed my face and head with it. Then she took me into the house. There in the house I saw some Ansari women who said, “Best wishes and Allah’s Blessing and good luck.” Then she entrusted me to them and they prepared me (for the marriage). Unexpectedly Allah’s Messenger came to me in the forenoon and my mother handed me over to him, and at that time I was a girl of nine years of age.

Sahih al-Bukhari 3896,
Narrated Hisham’s father: Khadija died three years before the Prophet departed to Al-Madina. He stayed there for two years or so and then he wrote the marriage (wedding) contract with Aishah when she was a girl of six years of age, and he consummated that marriage when she was nine years old.

Sahih al-Bukhari 5158,
Narrated Urwa: The Prophet wrote the (marriage contract) with Aisha while she was six years old and consummated his marriage with her while she was nine years old and she remained with him for nine years (i.e. till his death).

Sahih Muslim 3310
Aisha reported: Allah’s Apostle married me when I was six years old, and I was admitted to his house when I was nine years old.

Sahih Muslim 3311
Aisha reported that Allah’s Apostle married her when she was seven years old, and she was taken to his house as a bride when she was nine, and her dolls were with her; and when he (the Holy Prophet) died she was eighteen years old.

Sunan Abu Dawud 2116
Aishah said: The Apostle of Allah married me when I was seven years old. (The narrator Sulaiman said: Or six years.) He had intercourse with me when I was nine years old.

Muslims’ claim about Mary being still a child when married to Joseph cannot be substantiated by a single reliable source. Indeed, not even the Bible explicitly gives us her real age, but it actually gives us the notion that she was not a child like Aisha (9) when she got married, since the Bible calls a “12 years old”, a child:

35 While he was still speaking, people came from the synagogue ruler’s house saying, “Your daughter is dead. Why bother the Teacher anymore?”
36 But Jesus, when he heard the message spoken, immediately said to the ruler of the synagogue, “Don’t be afraid, only believe.
37 He allowed no one to follow him, except Peter, James, and John the brother of James.
38 He came to the synagogue ruler’s house, and he saw an uproar, weeping, and great wailing.
39 When he had entered in, he said to them, “Why do you make an uproar and weep? The child is not dead, but is asleep.”
40 
They ridiculed him. But he, having put them all out, took the father of the child, her mother, and those who were with him, and went in where the child was lying.
41 Taking the child by the hand, he said to her, “Talitha cumi!” which means, being interpreted, “Girl, I tell you, get up!”
42 Immediately the girl rose up and walked, for she was twelve
(12) years old. They were amazed with great amazement. – Mark 5:35-42

As we see, the Bible clearly shows us that a “12 years old” is considered as a child. Let’s now see, what does the Bible say about Mary? Does it refer to her as “a child” (12 years old or less)?

39 Mary arose in those days and went into the hill country with haste, into a city of Judah,
40 and entered into the house of Zacharias and greeted Elizabeth.
41 When Elizabeth heard Mary’s greeting, the baby leaped in her womb, and Elizabeth was filled with the Holy Spirit.
42 She called out with a loud voice, and said, “Blessed are you among women, and blessed is the fruit of your womb! Luke 1:39-42

As we see, Mary was categorized among “Women”, not “Children” [i.e. 12 years old].

Come to Christ, O Muslims! Stop fighting a lost fight.