Alexandra and Katharina Andresen are the youngest billionaires in the world with a net worth of $1.4 billion dollars each. Norwegian sisters are 21 and 22 years old. They maintain the title for the 3rd year in a row. While the youngest self-made billionaire title bagged by John Collison, who is 27 years old. 2018 witness increase in the number of billionaires under the age of 40 to 63 as compared to 56 in 2017.