Should I mention I was denied a student visa for USA some 6 years ago?
First off, you should always declare something if asked directly. Most visa applications, in my experience, have a question like, "Have you ever been denied a visa to any country in the last 10 years?" You'll need to answer this question honestly and fully, so include your unsuccessful USA visa if relevant. If there is no specific question for which this detail would be relevant, you're not obliged to volunteer it.