Buying iPhone from a carrier?

Is it possible to walk into a store like Verizon or At&t and buy an unlocked iPhone without a plan? Are there any restrictions like age or credit? Can I simply buy an iPhone with cash?

You have to get a plan if you walk into a carrier store.

If you want an unlocked iPhone then get it from Apple directly. I think Apple only takes credit, debit, or Apple pay.

Not that I'm aware. The best you could get there would be an off contract phone, though those are still locked to their respective carriers. If you want an unlocked iPhone you'd have to check an Apple Store or of course there are the online order options.