Many preterists (full or partial) hold to the two-age model of eschatology (the Jewish age ended in 70 A.D., ushering in the Gentile age). How do preterists reconcile this perspective with Jesus' promise in Matthew 28:20 that He will be with Christians "all the days until the completion of the age" (literal translation). Does that mean that Jesus is no longer with us since the age to which He was referring has ended from a preterist perspective?
Although I'm not a preterist (none of that is important to me), this is the argument you would need to consider.
It is natural for us in English to think of the conjunction ἕως (heōs), translated as "until," as the terminus ad quem of an action.
That is to say, most read this scripture and think, "Jesus will be with the disciples until the end of the age, but after that, he will no longer be with them." In other words, the "end of the age" is the terminus ad quem or "limit to which" Jesus will be present with them, and after the end of the age, he will no longer be present with them.
However, ἕως is a bit more flexible than simply pointing to the terminus ad quem of an action.
Consider 2 Samuel 6:23.
Mikhlal did not have a daughter until she died. Quite plausible, possible, and probable.
However, if ἕως points to "the day of her death" as the terminus ad quem, then we must admit that Mikhlal had a child after she was already dead.
Such is impossible. Clearly, ἕως does not always point to a terminus ad quem. Mikhlal did not have a child to the day of her death, and thereafter. Likewise, Jesus would be present with his disciples to the end of the age, and thereafter.
Not necessarily. Technically one could take the two-age model literally where one age would end in 70 (and thus Jesus would no longer be with us), but most who hold to this believe that the ages overlap. So Jesus was talking about the present age that overlaps the former age when he said it. Thus he will be with us to the end of the current age, which overlapped the former age.