[BitWielder1]

If before 18 or able to pay for it themselves, make them earn the privilege to have one, and make it clear that if they misbehave, it will be taken away.
Learn how to disable the phone remotely if they don't surrender it.
No replacement if lost or broken.
Availability of apps and features should be age appropriate.
Make them earn that too.

And after 18 and they get a phone and plan that they can't afford, don't pay it for them.
I'd avoid family plans, my kid used up the entire monthly data limit in a week, leaving nothing for the wife and I.

[MinorityRepublican]

If before 18 or able to pay for it themselves, make them earn the privilege to have one, and make it clear that if they misbehave, it will be taken away.

The only way to go, IMHO. At least they will be motivated to look for a job, if they get the privilege of buying a smartphone. Because they are responsible for their payments, they have to keep the job. Every parent should be doing this even if the family is well-off.