Well, I can't answer to whether it is safe or not to drink slightly older urine, but I can say that I've done it with no untoward results.
On the thread above, someone was asking how urine can be antibacterial and yet people develop urinary tract infections. The study I cited looks at that question, and it turns out that urine can have degrees of antibacterial activity.
I remember reading in a medical study or similar level of evidence--and it's so frustrating that I can't remember where--that people with chronic UTIs tend to have more alkaline urine. So acidity is important. I remember reading on one of the threads in this forum that someone developed infections of some kind (kidney? bladder?) once their urine became too predominantly alkaline. I'm going to have to go back and take notes... when I have time.... in another life.
Morning urine is both more acidic and contains higher levels of uric acid-- both antibacterial components of urine -- so it seems safe to conclude that morning urine could be kept a while and drunk later, that it would "ferment" more slowly, whereas urine from later in the day would "ferment" more quickly and be less safe to drink "slightly old". All theory though!