Physically yes (at least in late teens), mentally no. Their brains have not finished developing.
From a less biological standpoint, they're also still (typically) living with their parents and attending school, largely insulated from the real world.
But it's really the brain development that kills the argument. Any argument about whether a teenager is an adult is almost guaranteed to revolve more around the mental/emotional aspects of adulthood than the physical.
Let's stick to unpopular opinions, not incorrect facts.