How old is the youngest diamonds?

Answer 1

All natural diamonds are as old as earth itself -- within a range of countless numbers of years.

One could say, however, that man-made diamonds are younger than natural diamonds. Some were made yesterday.

Answer 2

Diamonds are formed deep within the Earth's mantle over billions of years, so the youngest diamonds available for mining are typically at least hundreds of millions of years old. However, there are some rare cases where diamonds have formed more recently, but these are still likely to be tens of millions of years old at minimum.