I think everyone had to agree over one number, otherwise there would be chaos. Most of us mature (in the sense, understand and accept responsibility) at 14 or 16, I guess. But that's most, not everyone. Since it is not feasible to measure each person's responsibility level, they may have decided to add a buffer of 2 years and fix it at 18.
Just a guess.
Not because you can get porn - but because you can BE IN PORN!
I agree - 18 is arbitrary. I think education and family values should be a higher weight.
We all know some 20 year olds don't deserve to be of legal age - and some 14 year old boys perhaps wouldn't turn into social rejects if they could use their allowance money to go find a prostitute. Instead of always having to hide their porn files on their computer - or get rejected by pretty girls at school : D