Another 2 cents worth...lol.
Longhorn bulls are just "coming into good age" at 5 or 6 years old. Some are still doing very well into their mid and late teens. Same way with the Longhorn cows. If a bull has good genetics, temperament, conformation, is registered, and producing quality calves out of a variety of cows, then why trash him if he is doing well just because he is getting a little older?
Every bull should be evaluated (for service longevity) based on what he can do. Our decision points are (1) at weaning and (2) following a semen test at 13 to 14 months. If both are "good", he is a keeper (based on a numer of evaluation factors) for either a Longhorn sire or for a commercial cross-breed sire. On the flip side, if a bull doesn't pass the test for (1) and (2) above, then he is either sale barn or freezer beef.