The "10,000 hour rule" is part real and part hype.
The reason it is part real:
If someone spends 10,000 hours doing something the average person will be pretty good at it. If they happen to also be naturally inclined in that area they will be phenomenal at it. If they are naturally gifted in that area they might be one of the best in the world at it.
Why it is some hype:
The rule tends to apply more to skill based activities like art, music, anything hand eye coordination related, some sports (golf for example)... activities that heavily rely on muscle memory. Your ability to perform at these activities is primarily dictated by how good your muscle memory is and there is no substitute for practice in obtaining muscle memory.
As you gravitate to activities that are either more pure mental (like math/physics) or pure physical (powerlifting, Olympic weightlifting, sprinting, etc) the rule starts to fall off. Practice is still important but your natural ability at those areas plays a bigger role.
If you aren't genetically gifted you can lift all the weights you want and you might get strong but you won't have a shot at even qualifying for the Olympics for weightlifting or sprinting much less winning. The same is true of the mental activities. If you spend 10,000 hours studying a subject you probably will have a pretty good knowledge of it but unless you happened to be blessed genetically you probably aren't going to be the Newton or Einstein.A little more about your question about when ability is shown. I don't know enough about modern mathematicians to be able to answer your question but probably not because of what I stated above. If a person is a well known mathematician they are probably naturally gifted at math which would have shown from an early age. This seems true in pretty much every field.