What do you call a person who started something that is later followed by everyone? For example, someone started to stay late at night till 8 p.m. and later everyone started to stay till 8 p.m..
someone who starts a trend, or makes one more popular
The other answers on this page have a couple other fine suggestions — and a thesaurus lookup turns up more suggestions still —, but be aware that some of them require additional qualification. For example, it's typically "a harbinger of something", or "a precursor to something". So you won't be able to just say "he is a harbinger" and leave it at that; likewise, "he is a precursor" will only raise questions if no further context is supplied. "He is a trendsetter", on the other hand, is perfectly self-explanatory and self-sufficient.
Maybe also initiator could fit?
one who initiates
Bellwether. Quoting from Wikipedia:
A bellwether is any entity in a given arena that serves to create or influence trends or to presage future happenings.
The term is derived from the Middle English bellewether and refers to the practice of placing a bell around the neck of a castrated ram (a wether) leading his flock of sheep. The movements of the flock could be noted by hearing the bell before the flock was in sight.
I felt the need to float "Innovator" given editor's suggestion of "early adopter", mostly to make clear that, within frameworks for understanding adoption rates, innovators are generally considered to be the first to adopt the innovation and are themselves followed by the early adopters. Innovators cast the die or blaze the trail, and early adopters may or may not follow. There's a media-related term (particularly in advertising/marketing/PR) "opinion leader" which is used as part of the theoretical construct for how new innovations spread. It may also be useful in this case for explaining the idea that people who aren't directly involved in the act of innovating are perceived as a more-reliable (i.e., impartial) bellwether by the average eventual adopter.
You could also, depending on your style and context, use the word precursor:
a person, animal, or thing that goes before and indicates the approach of someone or something else; harbinger
Two example sentences related to original poster's request:
- When I saw everybody's eyes on Jeremy as he glided across the hall in those "heelies" things, I knew he was the precursor to a new fad.
- Janette is the precursor to the new trend at the office to wear silly pins on casual Fridays.
Instigator might be appropriate in some contexts, if it's clear that others are now involved.
a person who brings about or initiates something
Avant-garde. From Wikipedia,
Avant-garde ([...] from French, “advance guard” or “vanguard”) is a French term used in English as a noun or adjective to refer to people or works that are experimental or innovative, particularly with respect to art, culture, and politics.
Avant-garde represents a pushing of the boundaries of what is accepted as the norm or the status quo, primarily in the cultural realm…
The last sentence suggests that the behavior of the avant-garde is eventually followed by at least some people.