How old would you have to be to be allowed to have a linked account to your parents?

umm don't think there is an age limit you just put the little one on your account and get them a new card for your account

I have had customers add their children as an authorized user to their credit cards as young as 12. They will only be authorized to use the card but will not be responsible for any balance legally unless they are added as a joint or co applicant and in that case they have to be 18.