The last time this happened was 2009 (mortgage-related crisis)
In a recession economic opportunities fade away, including the opportunity to make oneself a billionaire while still under 30. Thus the only people remaining on the list inherited their money.
Evan Spiegel ($3.1b, USA, 33, founded Snapchat) is now 33.
He used to be on the "billionaires under 30 list.
John Collison ($7.2b, Ireland, 33 created “Stripe” which allows everyday ma & pa Internet sellers to accept credit cards).
He used to be on the list.
Andy Fang ($1.2b, US, 31, founded “DoorDash”).
He used to be on the list.
Palmer Luckey (#2.3b, US 31, patented some of his teenage attempts to make a virtual reality headset. When the big boys became interested in VR headsets they paid him $1.3b for his patent rights)
He used to be on the list.