In my opinion this should be taken with a grain of salt. There may be a little bias too since YC obviously has an interest in the most stable of all configurations for their startups, which is the 50/50 split (although it may not be the most fair one).

I came up with the idea for the company Indeed a silly reason for unevenness.

I started working n months before my co-founder IMO if n > 6 it's a valid reason.

This is what we agreed to Well if it was a fair agreement among equals, why not?

My co-founder took a salary for n months and I didn’t Again, for n larger than a certain number IMO this is a perfectly valid reason not to split equally.

I started working full time n months before my co-founder See above.

I am older/more experienced than my co-founder Silly reason to some extent. All other things being equal a drastically more senior co-founder will likely have more contribution to the startup.

I brought on my co-founder after raising n thousands of dollars If n > 100 I think it's a valid reason not to split equally.

I brought on my co-founder after launching my MVP Depends on what the MVP is and how much traction it has.

We need someone to tie-break in the case of founder arguments That alone is a silly argument.

Note: Edited for formatting