Why would God do that? Why couldn't He just have let them have a baby at a normal age, like 20? There's so many answers, but the answer that encourages me the most is that God knows what's best. God looks at us from eternity, not just from our small viewpoint.
Here are some snippets from Streams in the Desert by Mrs. Charles E. Cowman on waiting:
It is not for us who are passengers,
to meddle with the chart and with the compass.
Let that all-skilled Pilot
alone with His own work.
Some glorious morn--but when? Ah, who shall say?
The steepest mountain will become a plain,
And the parched land be satisfied with rain.
Rough places plain, and crooked ways all straight,
For him who with a patient heart can wait.
These things shall be on God's appointed day:
It may not be tomorrow--yet it may.
What are you waiting for?