In most cases, if both the numerator and the denominator of a fraction are prime for example in the fraction 2/5 the greatest common factor of the numerator and denominator will be 1 which means that the fraction is in lowest terms.
However, if the numerator and the denominator are the same prime number for example 7/7 the fraction is not in lowest terms because the greatest common factor is the prime number in this case 7.
7/7 can be written in lowest terms as 1/1 therefore if the numerator and denominator of a fraction are both prime, the fraction is not always in lowest terms so the given statement is false.