Why shouldn't under 18's have a mobile phone?

Answer 1

Who says under 18's shouldn't have a phone. If they are involved in many extra-curricular activities they may need a phone so parents can keep track of them.

If your question is aimed at why under 18's can't legally BUY a phone, it is because in the USA anyone under 18 is considered a minor. By law, a minor cannot enter into a contract (they are presumed not to have the necessary lifeexperience and knowledge to make these commitments.)

If a store WAS to sell a phone to someone under 18, the person could then come back at 18 years old and give the phone back and demand a full refund on their phone purchase because they were not legally competent at the time of the purchase (under 18) to make such a commitment.