On the finale of the most recent season of The Biggest Loser, two of the contestants (both in their sixties) made a very similar observation: Age is not a factor. Both of these contestants, having started the season severely overweight, finished it 80+ pounds lighter. Quite a feat for someone at their age.
When we look at the Christmas story, their are two pregnancies which testify to the same. Elizabeth, who was an old woman, conceives by her husband. Now this is not a new miracle, as it also appears toward the beginning of the Bible when the nation of Israel is founded. Abram is promised offspring so plentiful that they are like the stars in the sky, yet he is very old. And in the same way God keeps his promise to Abram/Abraham, He keeps His promise to Elizabeth and her husband Zechariah.
At the same time, Mary, who was a young woman and Elizabeth's cousin, discovers she too is pregnant. Mary, however, is not yet married. This news, while not shocking today, would have been life shattering then. It didn't matter that Mary, a virgin, was pregnant through the touch of the Holy Spirit - not that of a man. Yet, instead of destroying Mary's life, this news fulfills it.
In both cases, Mary and Elizabeth were faithful to God. It didn't matter that one was too young or that one was too old. They allowed themselves to be available for whenever God saw fit to use them.