Article 9 of the Saudi Personal Status Law sets the general minimum age for marriage documentation at 18 years. Marriage contracts for anyone under 18 cannot be officially registered without a court order. However, the law does allow a court to grant an exception for those below 18 who have reached puberty, provided the court is satisfied the marriage is in that person's best interest.
For expats, Saudi law applies to marriages conducted on Saudi soil, meaning the age requirement is relevant if you are registering a marriage within the Kingdom. Non-Muslim expats have their marriage contracts documented by the relevant notary authority (Article 8), but the age threshold under Article 9 still applies to the documentation process.
If you have concerns about a proposed marriage involving a minor — whether your own child or another — you should be aware that Saudi courts take the minor's welfare into account. Expats are strongly encouraged to consult their home country's embassy and a licensed Saudi family lawyer, as marriages involving minors may also have legal consequences in your country of nationality.
This is general legal information, not legal advice. For advice on your specific situation, consult a lawyer licensed in Saudi Arabia.