Does big4 really need young blood (i.e. young guys and girls in their 20s) to do vouching work?

People in 30s and 40s has no more energy to do vouching because of commitment to family e.g. they need to take of elderly parents in 70s 80 and children in 0 to 10s

Why don't nurture more people in 20s?

No 20s want join ?

Then end up 30s still do vouching?