How advanced is the concept of prime numbers?

Speaking in terms of cultures / civilizations, how far along does one have to be to understand prime numbers? What are the prerequisites? Verbal language? Written language?

I ask because I'm working on a novel (sci-fi) in which a planet has been set aside to preserve a potentially sentient species. On this planet, there are observation stations. The doors to these stations need to be protected by something hard enough that it requires cognitive reasoning to overcome, but still be designed to be accessible once the species is able to understand the information contained inside.

I'm thinking of a combination lock where to decipher the combination, you have to be able to factor a number into primes. For example, the lock may be lock #21, and the combination would be 3-7.