When your mommy and daddy stop by sack up and own your plant. You are grown up right? or do they still pay the rent for you?
WOW very interesting question for a grown man.........
Ok upon further review i might have been too harsh on you. It was the mom and dad comment that got me. next time leave that out. Potraostdinner has it nailed.
Don't be an asshole dude. It's a legitimate problem. I'm 24 and live by myself and having my parents stumble into my grow room was my #1 concern - not because they would be dissapointed or mad - I already know they smoke, so they probably wouldn't even care. I just don't want to have that awkward moment where I have to explain what's with the lights, mylar lined room, pots, etc.. I would much rather just avoid ever having this experience.
To answer the original question, I would try to limit visits, when they do come over, try to keep them in the main area and out of your bedroom, closets, etc.. Also, I would install some kind of lock on whatever area you are using for growing - as a last resort in case they wander into your bedroom anyway. If they ask what's up with the locked closet/bedroom, etc.. just say it's for security and you dont have the key on you. It might look a little suspicious but it's better than having them walk in on your plants.
Parents can be nosey, even when you've grown up, and sometimes it's just easier on everybody to not let them find out. I know how it is.