Famous at 12 in the US? She was not even remotely famous. Venus was already on the cover of Newsweek, but Martina wasn't known until her WTA success and even then it was only in the tennis/sports-following community. What coverage she got did not go beyond the norm. If you weren't interested in tennis, it would go right by you. It was tennis magazines and a GQ cheesecake cover, the occasional (tied to the slams) sports columns, back pages of the sports section of the paper, and special coverage of Grand Slams and the biggest tournaments in magazines and papers.
What country are you from, gogoMaggie? "Nowadays when a 16-year-old beats a couple of decent players, it's quite big news." ? Here, a younster beating a top player doesn't get any mention beyond the tiny print results box in the paper and they get ink only in the limited coverage of tennis in the papers. No pictures. They get two or three sentences instead of one if they mentioned in conjunction with the loss of the famous person. During televised coverage of slams, they get covered as the shows hype the matches, but they only talk about the ones whose matches will be televised. You have to be into tennis and looking for information for more than that. (For women's tennis, anyway. Not men's.) Same thing with print media—slam or Miami only "give the people a summary of what's coming up" coverage.
Martina never got that much attention until she was beating Venus and Serena as people were interested in them. Now, some people other than tennis fans recognized her name. Not a lot, though, until Venus and Serena focused on her. They made her much more famous than she ever would have been.