This is the first I ever heard about age restrictions on check-in. Is it a US thing? In Bali, years ago, Amandari had a regular guest who was under 16. He used to go there alone to revise for his exams, flying solo from the UK. He told me once that he also stayed at The Mark (back in Rafael days) and this, too, did not appear to be a problem. Maybe the Platinum card helped.