should a 20 year old guy wear flip flops?

If he wants to. There is nothing wrong with it.

Anyone of any age can wear flip flops in the right locations since they are extremely casual.

Well ya - what else is someone supposed to wear when it's hot outside? Shorts with socks and shoes? Talk about dorky.

A 20 year old can wear flip flops if he wants to. I see guys wear flip flops even if they are in their 60s