Perhaps the only seats left were in an emergency exit row? Depending on the country and airline, passengers seated in the emergency exit rows need to be at lea

Perhaps the only seats left were in an emergency exit row? Depending on the country and airline, passengers seated in the emergency exit rows need to be at least 12 (EU, UK) to 15 (US, Australia) years old. The IATA definition of “child”, for the purpose of fares, is under 12. Thus, a narrowbody jet with 3-3 seating could have room for up to six adults but not a single child. That said, exit rows usually get snapped up quickly so this may be unlikely.