The San Jose Worldcon bid wants to crowdsource suggestions for Guests of Honor. It says that among "the traditional criteria for Worldcon Guest of Honor consideration" is "an established career, usually considered to be 30 years from entry into the field."
And I wondered, how long has it been 30 years? In the early days, the SF field hadn't been around very long, and because it was small, new names could easily make a big impact. I remembered that Robert Heinlein was GoH at the third Worldcon in 1941, only two years after he sold his first story. That would be highly unlikely to happen today, even for another Heinlein.
So I made a list of all the professional fiction writers who've been Worldcon GoH over the years. Just the authors, because the SF Encyclopedia is conscientious about listing first published stories, but it's not so rigorous with the entry dates of artists or other categories of pros. Making a quick chart, I found that less than 30 years was the rule up until about 1970, and, that among authors, only Hugo Gernsback (1952, 41 years since his first published SF story, but he was really honored as an editor, and it was only 26 years since he'd founded Amazing), Murray Leinster (1963, 44 years), and Edmond Hamilton (1964, 38 years) exceeded it, though a few others came close.
Since 1970, under-30s have been less common, though for many years they still occurred frequently (Zelazny, 1974, 12 years; Le Guin, 1975, 13 years; Ellison, 1978, 22 years; Haldeman, 1990, 21 years; and some others). But since 2001, there have only been two authors with less than 25 years: Bujold in 2008 (23 years), and 2017's Nalo Hopkinson (who will be 21 years at that point).
I also calculated the age of the GoHs, not at the time of their GoH-hood, but at what age they entered the field as professional authors. That average has remained unchanged over the decades; averages over ten-year periods consistently come up with age 25-28.