Yes - an 18 year old can seek asylum in a Canadian airport.
However it's a very bad idea to destroy your documents when you get to Canada. This will create challenges at the port of entry with confirming your identity and may complicate your asylum claim later. If you want to claim asylum, you should keep your documents and claim asylum using your documents and ID.
Agreed. There is no benefit in destroying any documents from your home country. It only adds delays in determining your identity and can result in you being detained until your identity is confirmed.