In the poem "Mending Wall" by Robert Frost, when historically is the speaker speaking? Is he or she reminiscing? Addressing present concerns?...

I would say that in this poem, the speaker is doing all three of these things, for the action described in the poem – the speaker and his neighbor rebuilding the wall that separates their properties – is a spring affair that happens each year.  While the speaker is thinking in the poem, the two men are rebuilding, and yet the speaker's thoughts travel to the same occurrences in years past and predict that his neighbor’s attitude toward the wall will not change in the future.

The poem begins with the speaker making note of two ways the wall becomes wanting in repair:  hunters come during the year and tear it down searching for their prey; and, more mysteriously, “Something there is that doesn’t love a wall,/That sends the frozen ground-swell under it…And makes gaps even two can pass abreast.”  Holes big enough for two – perhaps here symbolizing the speaker and his neighbor, who could take the opportunity in the wake of the wall’s crumbling to pass over and join each other, to each be fully in the other’s world – and yet instead, they fill these holes and remain separated.  Neither of them knows how these holes come to be, “but at spring mending-time we find them there.”  This is a recurring phenomenon, and so when the speaker speaks of it, he is speaking of both the past and the present.

The speaker does not want the wall there – it serves no purpose but to delineate the boundary between two properties – there is no fear of cross-contamination from the trees both of them cultivate, neither of them keeps animals – the fence is utterly meaningless, and only serves to separate the two men.  The speaker’s neighbor will not budge on this point, for, as the neighbor’s father often said, “’Good fences make good neighbours.’”  For this man, the object is not to become closer with the speaker and become friends, but to maintain a healthy distance.  And, for the speaker, the neighbor “moves in darkness,” with “…a stone grasped firmly by the top/In each hand, like an old-stone savage armed.”  The speaker is implying that the neighbor is set in his ways and maintaining meaningless traditions simply so that he will not have to change.  Near the end of the poem the speaker says that “he will not go beyond his father’s saying;” he will not change.  This is the only hint at a prediction for the future in the poem, but given the past, we can assume that the wall-mending will go on, as long as the neighbor maintains his property.