Re: When did you stop being a beginner in Aikido?
My understanding is that beginner is shodan-sandan.
Intermediate is yondan-rokydan.
Mastery is 7th+
Of course you have to maintain beginner's mind regardless of level.
I've seen a 7th dan who was attending a seminar that someone else was teaching and during lined throwing - the 7th dan checked out his interpretation of the throw with a 1st kyu. That rocks.
To answer the question posed, I think when you are legitamately yondan skill level - which by my way of thinking means you can use your mind and body in both a unfied way and also separately to achieve the desired tasks. Actual rank doesn't mean much - some get it based on loyalty which is fine but doesn't mean your not still beginner level ability.