Also, 123RF, DT and istock say 18. The issue would be the legal capacity to enter into contracts, which varies by country. Legal capacity is a person's authority under law to engage in a particular undertaking or maintain a particular status. If they can't legally sign a contract because they are not an adult then neither the site's contributor's agreement nor any model releases would hold up in court if a model sued for any reason. Parents or legal guardians would only be able to sign up for their under-18 children with special permission from (some) sites. You would probably have to sign some kind of agreement of indemnity. You should contact the sites that your son is interested in to inquire if you can't find that info on their sites. Each site posts their contributor's agreement and/or terms and any age restriction would be listed there.