​Strictly speaking, the Bible does not explicitly say, “Pray the rosary and other devotions to Mary.” But, of course, that’s a somewhat obvious answer. The real answer to your question has to do with the biblical foundation of those practices.  
 
There are numerous ways that devotion to Mary is found in the Bible. Here are some of those ways.  
 
First, the Rosary, which is the most common devotion to our Blessed Mother, is primarily Scriptural. On each bead of the rosary, we pray the words of the Angel Gabriel to Mary, as well as the words of St. Elizabeth to Mary. “Hail, full of grace, the Lord is with you.” And, “Blessed are you among women and blessed is the fruit of your womb” (Luke 1:26-46). Additionally, we pray the “Our Father” prayer, which is directly from Jesus, and the “Glory Be” prayer, which is echoed in Scripture many times. So, yes, the Rosary is biblical in that sense. We are primarily repeating passages from Scripture. Additionally, in praying the Rosary, we are to meditate upon one of twenty different Scriptural stories during each of the twenty decades. So the prayers are Scriptural and the meditations are Scriptural.  
 
Second, devotion to Mary in general comes to us from the fact that Jesus lived the Fourth Commandment perfectly. That Commandment says, “Honor your father and mother.” Jesus honored His mother perfectly, and we are to imitate Jesus in all He does. Therefore, we are to honor His mother, too.  
 
Third, at the Cross Jesus said to the Apostle John, “Behold, your mother.” And in giving Mary to John, the Church has always seen that Mary is given to all of us. We are all to behold our mother in the person of Mary.  
 
Fourth, Jesus gave Peter the Keys to the Kingdom of Heaven and said that whatever he binds on earth is bound in Heaven (Matthew 16:19). He gave Peter the gift of infallibly leading the Church into all truth. We believe that this gift of infallible teaching did not stop with Peter but was passed on through the ages to all his successors, the popes. The popes have definitively taught, through countless ages, the value of honoring our Blessed Mother. So though this last point is not a direct biblical call to honor our Blessed Mother, it is an indirect biblical command through the teaching of the Church founded by Christ in the Scriptures.  
 
It would require a book to detail all the ways in which Mary is spoken in the Old and New Testaments. Hopefully, the thoughts above help.

​These two questions are actually hard to answer with accuracy. In many ways, the answers remain a mystery.  
 
From a theological perspective, we can say that God chose Mary to be the mother of Jesus because she, herself, was the Immaculate Conception. This means that she was the only fitting mother for God in the flesh. Mary was conceived in the womb of her mother in a miraculous way in that she was conceived without sin. God chose to bestow upon her a “preservative grace,” meaning, God preserved her from all stain of sin, including Original Sin, at the moment of her creation in the womb of her mother. Of course, He did this so that she would be a fitting vessel for God the Son to become incarnate in her womb. The grace that preserved her came from the Cross of her Son Jesus, but it transcended time so as to free her at the moment of her conception. Thus, her Son was her Savior, even though He had not yet been born in time. If that’s confusing, try to ponder it for a while. It’s a great mystery of faith and a deep one also.  
 
Additionally, Mary chose to remain free from sin throughout her life. Just as Adam and Eve were born without sin, so was Mary. But unlike Adam and Eve, Mary never freely chose to sin throughout her life. This made her the perfect vessel for the Son of God. Her body and soul were perfect, making her the perfect instrument.  
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But this only answers your question from one perspective. You might also be wondering, “But why Mary?” This is a question that is hard, if not impossible, to answer. It’s most likely a matter of God’s mysterious will. Perhaps God, who can see all things and knows all people even before they are born, looked at all women from all time and saw that Mary was the one who would never freely choose to sin. And perhaps for that reason God chose to gift her with the Immaculate Conception. But this is ultimately a mystery of faith that will be revealed only in Heaven.  
 
As for your second question, “Why was she so young?” this might be easier to answer from a historical perspective. Today, in the twenty-first century, it’s uncommon for a fifteen-year-old girl to marry and have a child. But it wasn’t that way back then. When Mary had Jesus, she was seen, not as a dependent child, but as a young woman ready to start a family. So it’s always important to try to understand the culture of the time when considering questions of history.

​The short answer to this question is “No.” Let’s explore further.  
 
Most likely you are asking this question because some Scripture passages seem to suggest that Jesus had brothers and sisters. For example, here are two such passages:  
 
Mark 6:3– Is he not the carpenter, the son of Mary, and the brother of James and Joses and Judas and Simon? And are not his sisters here with us?”  
 
Matthew 13:55-56– Is he not the carpenter’s son? Is not his mother named Mary and his brothers James, Joseph, Simon, and Judas? Are not his sisters all with us?  
 
Should we conclude from these passages that Jesus had brothers and sisters in a literal way, meaning Mary and Joseph had other children in addition to Jesus? Certainly not.  
 
Below is a brief answer to this question that is taken from Chapter 3 – The God-Man and His Mother of the book My Catholic Faith!  
 
The Virgin Mother  
 
Scripture is quite clear that Mary conceived Jesus in a virginal way – by the power of the Holy Spirit. Additionally, it is the constant teaching of the Church that she always retained her virginity. She had no other children and never engaged in the marital act with Joseph. Sometimes this is confusing since there are references in the Scriptures to Jesus’ brothers and sisters. But this language was commonly used in reference to cousins. This shouldn’t confuse us if we understand the language of the time. In fact, this language is still a common practice in other cultures of our day. People from various African countries commonly refer to those within their own village as their brother or sister when they are with them outside of that village.  
 
God’s plan was that Mary have one Son for various reasons. It was especially so because we are all called to become brothers and sisters of Christ through spiritual adoption. Therefore, if Jesus had had blood brothers or sisters, it would have undermined his spiritual brotherhood with them. It would have made their relationship with Jesus different. God desires this deeper spiritual relationship with all people. Mary is the only blood relation to Jesus; thus, she shares a special bond with Him that no one else does. It had to be that way if God were to become true man. Interestingly, even Mary’s deepest bond with Jesus does not come as a result of her physical blood relationship. It comes, first and foremost, by her spiritual motherhood. She is united with her Son primarily by her perfect faith and obedience. It is this bond that surpasses the natural one.