First of all, no, they're not adults. Kids can have phones, too.

Second, that's an extremely weak argument and I question if it's being made in good faith. But in case you are: the responsibility is on the manufacturer to make a safe product. That's why we have regulations on consumer product safety. You can't sell certain dangerous chemicals. You can't sell plastics with BPAs in them. Et cetera.

The consumer, when they have the product, also has their own responsibility to look out for their own safety. But that doesn't make it ok to give them something dangerous, especially if it's something that contains dangers most people don't even understand or know about, like malware. (Most people are "aware" of malware, but they have no idea how it works or how to protect themselves from it.)