I doubt there's a law, but it take a brain-dead agent to bump minors.
Did they tell the agent in MAD that they were all under 18?
I called my daughter and asked her to hand the phone to the agent. Once she got on the phone I asked her how she can bump my children who are ages 11,15 and 17. She said it is because they are one of the last ones to check in and according to the law it can be done. I said let's put the law aside how about your conscience? She then handed the phone back to my daughter and said " I cannot speak to your mother".
My children were then sent to another area in the airport to get some sort of compensation but then the Iberia lady said to them that since they are all minors it is against the law to give them a check. I asked my daughter to hand the phone to this agent hoping for some compassion, but as soon as I said I don't understand if they are too young to be compensated then they are too young to be bumped, I was once again told that they cannot speak to me.
I felt so helpless and frustrated.