Well ... I think it would be almost impossible for someone to call themselves a reptile/amphibian expert. There are way too many species with way too many different needs. Knowledgable - certainly - but reptile keeping is part science and part art. Take chameleons (guess what's my fav???) for instance. You can research yourself to death (which I do) - but you have "to become one with the chameleon" (which I'm working on) to truly give expert care. I think that would apply with all species. And that takes alot of time (years) and energy. While I'm sure younger people can be excellent keepers - I think the years of experience are necessary to fine tune those skills.
Raising a species, sucessfully breeding them and raising the neonates to adulthood could be used as a description of expertise. Or keeping several individuals of a species for their entire lifespan.
Experts have had the time to go through the difficult parts of reptile keeping (the diseases, reproduction, aging, neonates, etc.); have had the time to learn from their mistakes and have had the time to learn from others mistakes. And they never stop learning and questioning.
I don't think it is a good idea that so many classify themselves as 'experts' - it can lead to people trusting advice that isn't warranted. I do agree with Laurier in saying that many of those that classify themselves as experts are probably intermediate (and there is absolutely nothing wrong with being novice, intermediate or expert - but wrong to present yourself in a false way).