I draw that line near the "very" late 60's to very early 70's. Federal Homesteading was still open, however, (the "quality" of the land options was greatly restricted) by late 60's. There were still good T&M sites (15 acres) available (Trade & Manufacturing sites) for guiding or trapping, commercial fishing operations. Everything changed with the Fork-an pipeline construction.

We were then harvesting Alaska Mountain Goats, (two legally) for food up at the end of mineral road, out of Valdez, Alaska, every December.

I guess the line could be drawn marking the end of WW-II and the public access to the Alcan Highway.

ALASKA is a "HARD COUNTRY for OLDMEN". (But if you live it wide'ass open, balls'to the wall, the pedal floored, full throttle, it is a delightful place, to finally just sit-back and savor those memories while sipping Tequila).