I think avoiding injury isn't about avoiding limits, but about taking care of yourself as those limits are approached, and indeed as those limits are moved to another level. Training, bike fit, pedaling technique, recognizing and responding to warning signs, staying hydrated, staying warm in the cold, cool enough in the heat, being rational about increases in mileage... these are how to stay out of trouble. Sure, physical limits will come down with age but I don't think it's necessary to avoid limits.