I tried to apply for a Supplementary Card on my American Express Aeroplan Plus card today. I wanted to get one for my son for emergencies since he will be going away to university next fall. I wasn't able to get one because he isn't yet 18. Apparently there is a minimum age of 18 to get a supplementary card. He won't be 18 until after he has completed his first year of university. I understand there are other options for getting him a CC for emergencies, prepaid, etc. I'm just not sure why they need this rule in place for a supplementary card since it would be totally secured by my card anyway.