That's odd that airlines won't offer it to you at under the age of 18. I've had my United mileage Plus card for much time now. I think that if you go under your parents name and at one point in time choose to switch over to your sole FF
plan you will either need to buy the miles from your parents or they can give them to you as a gift, now there is a limit to how much they can give you... I, for one, have never had this issue. I get my card each year in Feb. and some times it changed due to how many miles I flew in a year, I think the most was like 100k, this year its 50k. I can check my miles under the FF
plan @ United.com and when I want to buy flights I can do so solely. My parents have their main account for the two of them with their credit cards attached. However, my sister whom is in collage has her own sole FF
plan but her credit card is still attached to my parents FF
I guess it has to do with what airline and the standards and other such systems/programs they have in place.
Thanks and good luck!