We gave our daughter her own phone when she was 10. She’s always been smart and responsible.
Nothing magic happens when they turn 18 except that they’re emancipated adults.
Why would you buy an emancipated adult their own phone?
The only thing you should buy your 18 year old child is luggage.
It is not about money. It is about human development. They need to learn to socialize in their teens. They’re in a different phase of development at that time. People need to learn to speak face to face before they can manage alternate forms of communication