We are faced with all kinds of challenges as we grow older. As infants we're tasked with the impossible feet of ambulation. As adolescence we're tasked with dealing with meaningful social interaction, and as we pass into our teen years we are charged with the monumental feat of self identification, and separation from the parent/ offspring relationship, and graduate into adulthood as a self reliant, responsible, young adult. It's these tests that help develop our personalities and define our character.
Unfortunately for you, your situation is more touchy only because of your age and reliance upon the social structure of your home and reliance upon your parents. Naturally, my usual advice to another adult to just dig in, and force others around to accept them for who they are ultimately won't work in full here. You still live with your parents, and you should be respectful of that. However respect is a two way street, and they need to be more sensitive to your hobbies or interests.
In respect to the larger picture here, you've got it easy. All things considered. Some kids battle their families over addictions to drugs, or alcohol. Others try to get by with teen pregnancy, or bad choices for social structure outside of the home. Others have dysfunctional families that are as shattered as glass mirror dropped off a ten story building. You, so far as I can discern from reading this short message, just have a family that doesn't jive well with your desire to perfect your art.
I think what you are experiencing is relative to each of us in the magical arts. To a degree, I think we have all gone through a little bit of what you're facing right now. When you have a passion for magic it is something that goes with you. Anywhere you go, cards, and coins will follow. I can recall the countless arguments I've had with my wife over caring my coins and cards with me when we went out on dates, or to theme parks. Some of those discussions even turned quite brutal as they would lead into full blown arguments. The arguments were always "This is what I am" vs "I wanted this time to be our time, not our time plus the cards." A good friend of mine had his wife tell him it was the magic or her. He's currently happily divorced. Guess which one he chose?
The thing that your family does need to keep in consideration is a two fold thing. One, that what you are in love with is an art form, and the other being that it's perfectly healthy thing to be involved in. It builds creativity, social skills, and fine motor skills. For the most part all you need to be involved is a deck of cards which can run between three and ten bucks, and learning DVD's which retail between twenty and fifty dollars usually. Like all hobbies it does require disposable income to maintain, but that's what they have summer jobs for. This also teaches you the importance of money, and that you need to be willing to work for what you want. Also two very important skills for you to have. I could also mention the importance of learning how to balance a check book too, and other banking related skills at this time, but i think it would be over kill. If you are able to regurgitate a fourth of this to your parents, or better yet just let them read this for them self, I'm sure my points will ring true. I would be interested to know if they would feel the same way about any other art from you would get involved with, be it music or drawing, modeling, or even acting? When you feel so strong about a hobby that your love turns to passion it's very hard to leave it at the door when you leave the house, if only because you begin to see inspiration everywhere. I guess to a degree, your parents are right about your hobby being a waste of time. All hobbies are that by definition: A time sink. However I'm not sure that is a bad thing, that is unless you are obsessing about it to unhealthy levels. However being as I don't know you I am in no position to make that kind of assessment. again, I am only reacting purely off what you have given me to work with here.
Ultimately I believe the time will come, perhaps sooner than later, for you to sit your family down and have an honest heart to heart with them. I believe you need to express your concerns regarding their stifling behavior around you, and how you feel that it isn't conducive to your growth in this hobby of yours. Problems can not be solved unless they are addressed, no matter how scary the concept may be, nor how adverse the reaction may seem. Your family needs to know that this is a hobby that you are serious about, and an art from you want to perfect. You all need to mutually agree upon acceptable terms for your involvement within this hobby that everyone can agree upon. They, to encourage and enable you to grow while you are at home, and you (perhaps as only a suggestion) to leave the cards at home once in a while to enjoy time out with the family. Perhaps they will help you set up a checking account complete with debit card, that they can closely monitor to help enable you to put hard earned money into, so that you can take responsibility for your own purchases from sites like Theory11? I don't see this as an impassible situation, only one that needs to be addressed, each parties concerns carefully expressed, and compromises reached. It's just as silly of an idea for you to just give up on something you are passionate about as it is for them to completely give into your desires, and change the way they live their lives with you without having any input to the matter. Remember that entire adulthood thing at the beginning of this post? Part of coming into your own as an adult is learning how to compromise with your family. Be in that meaning your parents, and siblings, room mates, or your spouse and offspring.
I wish you all the best,
Draven