I gave up on caring much about lag times a long time ago - just seemed to not matter much. I never put a correlation together on using old yeast, but it makes perfect sense. What I have found to be interesting is how little yeast slurry is needed when you are repitching directly from a first generation re-pitch and how quickly that gets started. I find that to be shorter than when using a starter, though I have not measured it specifically - just my impression.