I'm over 40 and would love to work a 30 hour week. But there is a cost for hiring each employee and so it's better to have fewer employees working longer. The cost includes interviewing and recruitment costs, payroll systems, management time, IT systems (email, systems access, hardware and licenses).
If we could slash all these costs and make it easier and cheaper to hire people, the 30 hour week might work. It might also make it easier for companies to fire people though, so less good for the 'youth'.