It would seem that Robert Frost was a supporter of the Modernist movement in that he is considered to be one of the foremost poets of this movement. According to biographer Tejvan R. Pettinger, at the poetseers.org website, the biggest difference between Frost's work and that of other Modernist poets (e.g., T.S. Eliot and Ezra Pound) is that he wrote on his own terms, rather than following the traditional or "popular" style of that movement.
Perhaps the very fact that Frost chose to do things "his way" was the closest he could come to the Modernist movement—resisting outside forces, in order to retain one's autonomy. (This might raise the question as to whether Frost was intentionally a Modernist poet or simply a man whose nature drove him to "march to a tune" only he could hear. I am not sure we can know the answer to this.)
In pursuit of personal autonomy, Frost was more traditional in the poetic forms he used. For instance, in his poem "The Road Not Taken," Frost leans toward using four or five beats per line. His rhyme scheme is steady: a-b-a-a-b. His tendency to use a more traditional meter (beat or rhythm) may have been in part due to his pleasure in reading his poetry aloud, rather than simply delivering his work in print.
Traditionally in poetry, verse is meant to be read out loud because of its musical quality. In trying to achieve this sense of the musical, poets will often use onomatopoeia, as well as assonance, consonance and alliteration: these devices create sounds that the "ear" picks up more readily than the eye.
I believe that Frost was far from being considered "new age," as the Modernist movement must have appeared to those more rooted in the traditional Realist movement. If the Realist movement of literature tried to provide writing that was "undistorted by personal bias" [http://en.wikipedia.org/wiki/Realism_(arts)], this would never have worked for Frost: his "bias" was his personal response to the world around him, and this element in his poetry may be what makes so many people believe Frost to be the greatest American poet in our literary history.