At the same time I set an arbitrary age limit for the group poets who were not yet born when I started publishing.

I understand he is going to refer to the group of poets born after a certain date, therefore restricting himself to poets of a certain age, and that age limit is arbitrary.

An option. He says he first published in 1980, so he's referring to those born after this year, I presume.

Spanish term (edited): (yo) trazó la frontera de la juventud

Así que (more of a Lat. Am. construction cf. ainsi que) también de manera arbitraria > Thus it was that, arbitrarily at that, I plotted vs. skirted or pushed the boundaries of (my own) youth.

I tend to share Toni C's hunch that there is an accent on trazó

Otherwise, the author's (obscure) simile or metaphor ought to be left literally for the reader to interpret.

"Voy a elegir el año 1980 como fecha de partida, porque esa fue la fecha en la que publiqué mi primer libro. Así que también de manera arbitraria trazo la frontera de la juventud en el grupo de poetas que no habían nacido cuando yo empecé a publicar."

"I am going to select the year 1980 as my starting date, as that was the year I published my first book. Hence by the same token,(equally/similarly) I am using that very same year as a marker for the younger generation of poets who had not yet been born when I first published"
It can only be " trazO" ,1st person singular present tense.Impossible it could be " trazó"; the clue lies in the very first word "VOY a elegir" "I am going to..", and there is no switch from the first person narrative to a third person narrative anywhere. What is at play here is a literary device common in the Romance languages I'm acquainted with whereby you use the simple present tense to describe an action which in English would be the present or past continuous tense.
"Yo trazÓ", Mr Melmann, really?? It would have to be" yo traCÉ", in the 1st person singular preterite tense. "cf. ainsi que "??? Why, that means "as well as" (tan bien como in Castilian),( and FYI "anziché" in Italian doesn't mean the same as 'ainsi que', it means ' instead of', 'in place of'.)'Así que' = 'hence', or as you say, 'so it is/was that', but that said, it is universal in Spanish and does NOT represent any particular variant. It is a metaphor ( a simile has to be introduced by 'as' or 'like' in English), and what the writer is doing here is a bit of whimsical musing and grandstanding, in a friendly and avuncular manner, with the subliminal message to the younger generation of poets that "this is what I had achieved well before your age, and on the timescales mentioned, (this generation are at least approaching 40, if not 45 as it does not specifically refer to those people born exactly in the year 1980) so I am laying down a marker/ yardstick/benchmark based on age but this is what you should be aspiring to achieve".