Why is it that adults preach about democracy and how great it is when really if
This might be taking your question too narrowly, but how about this: democracy is a form of government that places political power in the hands of citizens through their right to vote. But not all citizens are given a vote: five-year olds aren't, the mentally deranged aren't. In particular, if you haven't reached what used to be called "the age of reason", you are denied a vote. So, if your parents are all gung-ho for democracy, but insist on grounding your 15-year-old self on a Saturday night, well, that might be really irritating, but it's not inconsistent.
Parental rights over children usually lapse at the same time that their child acquires the right to vote. Coincidence? No. Parents have a right and a duty to make decisions for their children until they have reached a level of maturity and intelligence at which they can be held responsible for their decisions — and once a person has reached that level, democracies should extend him/her the vote.