I've been re-reading White and Zainasheff's Yeast book, and they mention more than a few times how you should only use yeast if the viability is over 90%. From the MrMalty calculator, it looks like liquid yeast would have to be made within 10 days of your use to have 90% viability.
So, my question is, for people who brew a lot of the same recipes of beer with the same yeast, have you noticed any difference in the finished beer if your yeast was one week old, rather than two weeks old, or three weeks?
I've used yeast that's pretty old. I recently used some yeast made in April, and the beer turned out fine. Maybe it could have been better with fresher yeast, but I'm curious how much of a difference it actually makes.
I wonder if they're just talking about direct pitching without making a starter?