Milestone Is a Marker in Someone's Life.

We use milestone to talk about important points in someone's life. We have the word mile, and we have the word stone. Now you put them together, you have a mile stone. And a milestone is a marker that used to be on the side of the road,. which would tell you how far it was to a certain place. So we're not talking literally here about a stone with 70 written on it for the queen. It's metaphorical.