Why "age && age >= 18" instead of just "age >=18" ?

I understand how in many cases it's important to check if a value exists before trying to do anything with it... and at 6:08 in this video Mr. Chalkley says:

If the number represents an age, and is greater than or equal to 18, we want to return true. If no number is passed into the function, or the number is less than 18 we want it to return false.

This is accomplished with the original function:

function isAdult( age ){
   if( age && age >= 18){
       return true;
   }else{
       return false;
   }
}
console.log( isAdult(33) ); // Returns True
console.log( isAdult(9) );  // Returns False
console.log( isAdult() );   // Returns False

However in the improved function this behavior is changed and instead, when no number is passed it returns undefined;

function isAdult( age ) {
   return age && age >= 18;
}
console.log( isAdult(33) ); // Returns True
console.log( isAdult(9) );  // Returns False
console.log( isAdult() );   // Returns Undefined

My Question

Is it better to have the function return undefined rather than false? Or in a real world situation would return age >= 18; be completely fine? (Which would return false if no value is passed, as in the originally stated plan.)