I've often wondered about the pimple thing myself. However, I don't think you can really rely on this. If you have a male and female that pair off and start laying eggs, you'll obviously no they have matured. if you have two Oscars are not going to pair off when you may not see any signs of sexual maturity, apart from when the Oscar starts digging around in the substrate. if you're only keeping one Oscar, the maturity doesn't make any difference to its life.
They say the Oscars reach maturity between one, and 1 1/2 years old. they should be a minimum of 7 inches at one years old, it really does depend on how it's being looked after. My Oscars have never started laying eggs before they are 1 1/2 years old.