Credit Card Lounge

What is the possibility of someone under the age of 18, such as a 17 years old, obtaining a credit card by just applying as a 18 year old? Also, if you happen to do that and get found out what would the consequence be, would they deny you acceptance in the future if you do this? This question has been in my mind for a while but no one seems to know the answer, any help would be appreciated.

Whenever I get credit card offers with bonus points, I open the card, purchase something small (like $10), get the bonus points (towards a gift card or something), and then I never use the card again. (of course I pay it off).
Does that have any consequence on my credit history? Should I close that credit card account, since I never use it again after I get my $100 gift card

Thanks in advance.
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