Otherwise:
a) Pre-idea
b) Post-idea, but pre-demo
c) Post-demo, but pre-validation
d) Post-validation, but pre-launch
e) Post-launch, but pre-funding
f) Post-fundingI've only had experience working with my one childhood friend whom I've co-founded all my projects with. We've worked together since we were kids, building dinky static web pages with Dreamweaver. So I've never had the experience of bringing on another co-founder, or working with someone I didn't have an extensive history with.
The reason I think its A) is because its not quite as important as to WHEN your co-founder joins, but WHO you co-founder actually is. A friend or someone you've worked with in the past is someone you can trust. Someone you can trust is someone you can bounce ideas off of and have "chemistry" with. The sooner the two of you have your brains firing off and inspiring each other with a concept or product, the better. This means you can argue sooner, and shape out what it is you want to build far more effectively. Ideas will constantly change, but the person you're stuck with will not. So the sooner you find the right person, the better!
The question of when will depend on a number of variables that you can only answer depending on the specific situation.
More importantly, ask yourself how much risk will the potential co-founder be taking on? In stages a-b-c there's plenty of risk; after f; there's not so much. If your potential co-founder is unwilling or unable to assume a significant amount of risk, relative to yours, then they're more of an employee than a co-founder.
Something you'll discover is that there are plenty of people who want the "co-founder" title but aren't willing to assume a commensurate amount of risk. They'll tell you to go out and get the money, then they'll jump in once you've guaranteed their paycheck. Avoid these people.
It depends on who this cofounder is and what role he is going to play. Does he have a narrow skill or market experience that is useful for a specific idea? Then post-idea. Is this someone with whom you've had rapport for a long time and many times said "hmm, it would be nice if we worked together on something"? Then pre-idea.