It is important to recognise what some translations will interpret verse 27 as 'engaged', but would have been something different to Jewish society. Parents or a matchmaker would have brought the boy and girl together as a good match very early in their lives. Then later, they would both meet for the first time and if they decided that they both would like to continue then they would become betrothed for one year when they would get married. They would be considered together from when they were betrothed, but wouldn't live together or sleep together until marriage. When Mary became pregnant by the power of the Holy Spirit, Joseph could have demanded her stoning!
The angel Gabriel visits Mary and tells her the news. Mary questions how this could possibly happen and the Angel gives her a rather oblique reply, which she accepts readily and recognises immediately the enormous privilege she has been blessed with. I have often compared her reception of the news to Zechariah's a few verses before-and plainly Luke wants the reader to do that-and it appears to me that Zechariah rebuffed the angel on far less surprising news than Mary had received in a situation where miracles and Angels might appear easily! If he had read the Old Testament he would have read of many such instances of women becoming pregnant after years of inability to conceive through the will of God. No one had ever been in Mary's situation before!
Now stop for a minute and THINK! Imagine what it would have been like to have been Mary. An Angel has appeared and given you the news of the months to come no longer filled with planning for a wedding, but full of pregnancy, being the subject of gossip and possibly worse. But look at what Gabriel tells her about her son: ''He will reign over Israel forever; His Kingdom will never end!''
Maybe we have certain lines that we are confident our lives will follow, but even if you are on older person, God may well have plans for you which are very different to what you are reckoning! Remember Elizabeth, but also Moses, who was eighty when he was called by God to lead Israel out of Egypt!