Under the Saudi Civil Transactions Law, a person reaches the age of majority at 18 years, calculated according to the Hijri (Islamic) calendar — not the Gregorian calendar most expats are used to (Articles 2 and 12). This means the effective Gregorian-equivalent age may vary slightly, so it is worth confirming the exact Hijri age when legal capacity is in question.
A fully competent person is defined as someone who has reached majority age, has full mental capacity, and has not been placed under interdiction by a court (Article 12). Only fully competent individuals can independently enter into binding contracts, make financial commitments, or manage their own legal affairs in Saudi Arabia.
For expats, this is especially relevant if you are dealing with young adult dependants (for example, a 17-year-old child on your iqama). Even if they are considered adults in their home country, Saudi law may still classify them as partially incompetent (Article 14), meaning certain transactions they enter into could be legally challenged. Always consult a licensed Saudi lawyer before allowing a minor dependent to sign any formal agreement.
This is general legal information, not legal advice. For advice on your specific situation, consult a lawyer licensed in Saudi Arabia.