I don't think that longer tails specifically are equated to older stallions. I have seen longer tails on different stallions some of which are younger - in fact a significant difference in sperm size in some stallions.
What one is more likely to see in older stallions is altered morphology - changes to the head, midpiece and tail. It is not going to be universal, and you might see little difference in some stallions, while other stallions are infertile because of the changes. Typically there will be a greater degree of morphological change from the age of 13 up, but I stress again, it is tremendously stallion-individual!
Another issue that is going to be more likely seen in older stallions (and again, by older I mean 13+) is an accumulation issue where there is not a daily shedding of dead sperm in the urine. This can result in a huge build-up of dead sperm that are then released at the time of ejaculation when collected. the end result is significantly lower motility (perhaps 1% total in the worst cases). These will also usually manifest as a huge number of separated heads and tails. Concentration may well be in the billions/ml as well.
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