Which of the following is NOT an element of a valid contract?

In contract law, the essential elements required for a valid contract include competent parties, a legal purpose, and consideration. Competent parties means that the individuals involved in the contract have the legal capacity to enter into an agreement, while legal purpose refers to the requirement that the contract's intent must be lawful. Consideration involves something of value being exchanged between the parties, ensuring that both sides are gaining or sacrificing something pertinent to the agreement.

Length of service is not a recognized element of contract validity. It pertains more to the duration of an agreement or employment rather than the foundational components that establish the contract itself. Therefore, it does not impact the legality or enforceability of a contract, which is why it is the correct answer to the question about what is NOT an element of a valid contract.