I agree with the notion of “age-lasting” or “pertaining to an/the age” or “age-during” – I note these are as typically expressed in ‘Young’s Literal Translation’ when it comes to rendering what is usually translated as “eternal”.
For mine, I understand eternal in qualitative terms expressing the idea of fullness or TOTALITY. IOW… “eternal life” speaks more to the quality and fullness of life IN THIS LIFE that is manifest by the reality of Christ; and thus we imbibe as the experiential consequence of knowing God through Christ, as per the following:
Jn 17:3 And this is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.
Jn 10:10 The thief does not come except to steal, and to kill, and to destroy. I have come that they may have life, and that they may have it more abundantly.
IF by “eternal life” Jesus meant LONGEVITY or ENDLESSNESS of life Jesus would have used a word like ἀκατάλυτος akatalutos as used in…
Heb 7:16 “who has come, not according to the law of a fleshly commandment, but according to the power of an endless life.”
So it is then that the “pertaining to an age” spoke of the THEN soon coming AGE they were about to attain in terms of covenant renewal, NOT an esoteric or ethereal “heaven” after death, rather it was the fullness of the new covenant AGE begun in Christ, i.e., “if any man be in Christ he is a new creation” – thus the new creation of Revelation is none other than the new covenant wrought in Christ.
Further to this I like for example N.T. Wright’s understanding and rendition of “eternal life”…
There is no-one who has left house, or brothers, or sisters, or mother, or father, or children, or fields, for my sake and the gospel’s, who will not receive a hundred times over, now in the present age: houses, and brothers, and sisters, and mothers, and children, and fields — with persecutions; and, in the coming age, the life of the new world.
N.T. Wright: Jesus and the Victory of God p. 402
In the NT the “present age” was the then Mosaic “world” or era of law-righteousness, with “the coming age” wherein dwelt “the life of the new world” being the new covenant age — the age from Christ’s cross forward. The real inter-testamental age then was the 40yr overlapping period Ad30-70; an age emulating Israel’s exodus out of Egypt [bondage] where after the pattern those of old, that is, those of faith, would come into the fullness of Israel’s redemption.
IOW, they grasped “the life of the new world” IN THIS LIFE which was none other than the “eternal life” as again defined above, that is, as a PRESENT reality… “life to the full” aka PEACE as per Jn 14:27.