Doug Moo, in The Epistle to the Romans, writes:
Contrary to Jewish expectation, the Messiah has accomplished the work of redemption, the Spirit has been poured out, yet evil has not been eradicated, the general resurrection is still future, and the final state of God's kingdom has not been established.
In other words, the new era has begun--has been inaugurated--but it has not yet replaced the old era. Both ages exist simultaneously; and this means that 'history,' in the sense of temporal sequence, is not ultimately determinative in Paul's salvation-historical scheme. Thus, the 'change of aeons,' while occurring historically at the cross, becomes real for the individual only at the point of faith. The 'change of aeons' that took place in Christ is experienced only 'in Christ.' Therefore, the person who lives after Christ's death and resurrection and who has not appropriated the benefits of those events by faith lives in the old era yet: enslaved to sin, in the flesh, doomed to eternal death. On the other hand, Abraham, for example, though living many centuries before Christ, must, in light of Rom. 4, be considered to belong, in some sense at least, to the new era.