What is the support or arguments to indicate the age of the Virgin Mary?

On many occasions I have seen that the Virgin Mary is described as a 13 or 15 year old girl when she had Jesus Christ. The main argument I have heard is based on the fact that in the culture of Israel girls were married as soon as they were 13 or 14 years old. I feel that with this argument are confusing the Hebrew culture of biblical times with the customs of India, China and Africa. In the whole Bible we do not find that such a custom existed in Israel, on the contrary, we see episodes in which one realizes that the marriageable women were 20 years or more. In fact, if a man was not considered an adult until 20, a woman would not be considered an adult before that age either. In the Old Testament we see that men who had already reached the age of 20 were called to contribute to the offering (*Ex 30:14; 38:26*), to go to war (*Nm 1:3, 20*) or responsible and fit for punishment for their sins (*Nm 32:11*) So what argument, apart from the one indicated *(If I am wrong in this argument, you could also indicate why)*, is applied to designate the age of the Virgin Mary as an adolescent?