Old cows are ones that have nubs for teeth or no teeth if bought cows; for ones we raise we figure by 12-15 they are getting old. Again, we look at condition, the last calf they raised, and check mouths everytime we preg check. Have one now that had great condition, is ornery as all get out, hard to get in, raises a really good calf, and has no teeth and is preg again. My son laughs as he knows I hate that cow. But we have several of her daughters and they all follow mom's genes and do good jobs with their calves. I keep praying she will come up open. If she has any problems with this next calf, or loses condition, she is gone. Since she is his cow, he makes the decision. If he buries her is is his problem.
Most times a cow will get to the end of her productive life and her last calf isn't very good because she just runs out of milk or she gets real thin putting her all into the calf. Now, if they have few/no teeth and are preg, they go into a field with the other "last calf cows" and will get to calve and raise the calf and never see a bull again and when it is time to wean the calf they both go to town.
This cow looked good, great condition and was just going about her business. She was on the last calf list. Oh well....