When is it time to replace my trainer tire My trainer tire has cracks running along the length of the tire. They cracks seem to go in pretty deep, but don't go all the way around the tire. However it still has quite a bit of rubber on it and doesn't seem to be worn too much. I also don't think I have put that much mileage on it, but it is probably 5 years old, but only used for maybe 20-25 sessions.
I'm using a fluid trainer so I'm not really worried about being hurt, but I'm wondering if I should worry about it breaking and coming off the rim and causing some kind of damage if it gets caught in something.
I wouldn't hesitate using a tyre in that condition on the trainer, in fact that's exactly the sort of condition i might expect to start using a tyre on the trainer.
The small cracks likely mean the rubber is deteriorating and may be less grippy, but that's of little worry.  It's very unlikely there is substantial damage to the actual carcass of the tyre.
Finally, the absolute worst case scenario is a small split begins, the tube pokes through and bursts - however positioned where they are there is very little chance of the beads becoming unseated.  So maximum damage is the embarrassment of explaining to people you got a flat on the indoor trainer.