Nobody honestly will know for sure. It does vary a lot. If the oscars are not locally bred (which I don't see too often), then they are bred free reign in giant irrigated ponds in south america. The drag a giant net across the pond, collect a bunch of fish. The sort them and throw the smallest ones back. Oscar fry hatch in a day and a half, and they grow quick. Considering they had access to plenty of natural fry sized food in the wild, they probably aren't more than a couple months old.
7 to 9 months is way off tho. From birth it takes about a year and a half to reach 10-12" if they are fed well. A 9 month old should be about 6" or so.