“a prophetess, Anna.” That the text tells us that Anna was “a prophetess” indicates that she was openly known as a prophetess. Thus her reputation would give weight to the words she spoke about baby Jesus.
It is an amazing demonstration of the love God has for His people that He would reveal to both Simeon and Anna that the Christ was in the Temple. The Temple was very segregated, with courts for the men, and courts for the women. The only way to get the word effectively to both groups was for God to tell both a respected man and a respected woman that the Christ was there.
“from when she was a virgin.” Stating it this way emphasizes the purity of Anna’s life, and simultaneously shows that this was her first husband. She lived with this man seven years, until he died and she became a widow; she did not take another husband, but remained a widow until she was 84 here at the Temple scene. See commentary on Luke 2:37 for controversy regarding Anna’s age.