Which of the following sentences is punctuated correctly?
Most college freshmen are eighteen years old, but on our campus they are as young as fourteen, and as old as ninety.
Most college freshmen are eighteen years old, but on our campus they are as young as fourteen and as old as ninety.
Most college freshmen are eighteen years old but on our campus they are as young as fourteen, and as old as ninety.
Most college freshmen are eighteen years old, but on our campus, they are as young as fourteen, and as old as ninety.