I am in a conundrum. Accelerator programs such as YCombinator and TechStars advise entrepreneurs to apply as early as possible for additional consideration. However they stress out that they are relentless to accept single founders.
I am a single founder. What is the best path? Apply early as a single founder and hopefully they pay enough attention to my application to consider recruiting a co-founder as part of my progress (when it happens) or wait until I have a great team and then apply as early as possible.
The short version. Don't wait until you have a co-founder. Apply now. If you are not accepted you can always apply later when you find a co-founder. Plus the practice you get from filling out the applications is valuable.
The long version. Although I agree with @Robin that you should apply to these incubators because there's no real drawback, my experience is that most of those bullet points don't apply in practice.
Our startup applied to both YC and TechStars (we were rejected by both). This has been my experience:
Both YC and TechStars get tons of applications for each class. We are talking high hundreds for TechStars and probably close to 1000 for YC - I don't have exact numbers. This means they can't spend a lot of time on each application. You have to grab their attention within the first few questions they read. They do not read every single answer on every single application. The point is that you most likely will not get any feedback from them.
A few startups may be lucky enough to get that kind of feedback, especially if they make it to the in-person interview portion. However, most startups will get nothing more than a rejection email with no specific reason. It's possible that smaller, lesser known incubators will give you that kind of feedback, but don't expect it from these two.
With that said, I do agree with Robin that you should apply to these programs now. In my opinion, the real benefit of applying is forcing yourself to go through the exercise of answering their questions. They ask some good questions that you need to be able to answer. We found that answering the questions on the applications was a very helpful exercise for us.
As you mentioned, they are very clear about the fact that single founder startups have a slim chance of getting accepted. One counter-example is Drew, the founder of Dropbox, who was accepted into YC as a single founder. Paul Graham has talked about how they accepted Drew into YC because of how impressed he was with him. So, as a single founder your probability of getting accepted isn't 0%, but unfortunately it is fairly close to that. I'm not saying this to discourage you from applying, I just want you to have realistic expectations.
Lastly, not getting accepted means absolutely nothing! They see a lot of great startups, but unfortunately they can't accept every one. Keep working on your startup, continue looking for a co-founder, and continue to apply to these programs.
Start talking to them now, there are lots of benefits and no real drawbacks.
A "No" early can be a good thing:
If they are worried about the single founder thing, you could approach them from a "just testing the water now" standpoint which they normally won't mind giving you some feedback, and you have taken the first step.
I wouldn't get a co-founder for the purpose of being accepted into a program like these. If a co-founder makes sense, adds value and you want one, then by all means, but do it for the right reason.
I can't help but feel like the goal here is to get into one of these programs rather than to build a successful product and/or business. Keep in mind what your actual goal is and take steps towards it.
Getting a co-founder for the sake of having a co-founder would be worse than going it alone.
As for when to apply, I agree with Robin's answer, why not apply.. you have nothing to lose.