This Q/A by G. Vos is so helpful for thinking about OT prophetic literature, eschatology, and the two-ages:
In many of these passages [Is. 2:2, Mic. 4:1, Acts 2:17, 1 Pet. 1:20, & 1 Jn 2:18], is not something entirely different spoken of than what we understand by “the last days,” namely, the New Testament dispensation of the covenant of grace?
Considered superficially, this is indeed the case. See, for example, Acts 2:17, where the outpouring of the Holy Spirit is spoken of as taking place “in the last days.” Nevertheless one ought to maintain that here, too, the eschatological meaning is present. The explanation is as follows: From the perspective of the older prophets, the coming of the Messiah coincides with the culmination of the kingdom, the end of all things.
Isaiah, for example, speaks in one breath of the return from exile, of the coming of the Messiah, of the end of the world, and unrolls all these events before our eyes as in one great scene. He sees only the peaks towering above everything. Accordingly, the older prophets reckon on only two time periods: “this age” (οὗτος ὁ αἰών) and “the coming age” (ὁ μέλλων αἰών). So, for Isaiah and for Micah the “last days” are the days that precede the end and at the same time precede the coming of the Messiah. The later prophets were granted in the Spirit to see more clearly how there would be a double coming of the Messiah, one for suffering and scorn and one in glory (Dan 7; 9; 12). Thus what in the older prophets was still combined or condensed into one coming was in the later prophets divided into two.
But now from this it follows as well that the time that elapses between the first and the second coming of the Lord can be viewed from a twofold perspective. If we fix our attention on the coming that is still expected and we include everything before that in “this age,” then we and all the New Testament saints live in the last days, that is, in the period that forms the eve of the second coming of our Lord in glory. If, on the other hand, we focus attention on the coming that is already past, and we draw the dividing line between the two ages at the first coming, then we in fact already live in the “age to come.” Consequently, since the time between the first and the second coming of the Lord is governed completely by the thought of His coming either as already having occurred or as still having to occur, one can call it “the last days.”