A term used in advertising
who identifies and espouses trends
before they reach the mainstream
This type of person is likely to be fairly young and have a high disposable income, which in advertising terms makes for a very desirable audience.
Those who create the trends are sometimes known as the innovators, those who don't notice something is trendy until half the world is wearing it/drinking it/doing it are mainstream, and those who only notice that brown is the new black when grey has become the new brown, are laggards.
A misprint in the media pack of my previous company claimed that our advertising reached "early adaptors", which I could only assume meant that our audience, rather than being hip young rich people, were large and cumbersome 1950's plugs.