I would consider someone an "expert" if they have been doing something for years and know the industry inside out.
Expertize skill = ordinary skill + extra skill, extra skill comes from lots of practices and experiences. I think when someone gets confidence on own extra skills which makes thing easy and successful, claims herself or himself as an expert. The same is true to those who claims themselves to be SEO experts.
YES, Success comes with more practice and experience in SEO. If we have good knowledge on seo by reading books and articles, without doing work on some different websites we won't get that experience and success.
A professional is someone who has been paid for the service they claim to be a professional in, but an "expert" really is only a title someone can apply to themselves without any real qualifications. I could claim to be a professional in the breeding of the Honey Bee, but that doesn't mean I'm any better at it than the next guy claiming to be an expert. I think the best thing to do is only consider someone an "expert" when their skills or talents are supported by other professionals in whichever field they are in. For example, a scientist is only referred to as an expert when they have been peer-reviewed and supported in scientific journals for their efforts in their field. An SEO "expert", in my opinion, is an individual or company actively supported and recommended by others who have used their services, or backed by a considerable figure in the industry.
One who is much experienced in SEO... Who has worked on many projects and gained success in taking them on top rankings... Who always keeps abreast with latest SEO changes and search engine updates and One who always keeps searching and finds new ways and techniques of SEO can be said as an SEO expert.