I'm going to go against the consensus of opinion here and say that I wouldn't even look at buying a cow that old. I have cows in my herd that old but I reared them from calves which is slightly different from buying in somebody elses old cow. They have already given me 10 calves, 1,000's of litres of milk, I know them and know what problems to look for and deal with.
While cows can and do live into their 20's, the average lifespan of a cow is between 13 and 15 years of age. By that stage they are often showing foot, leg, hip, spinal and udder problems and the costs of keeping them in good health outweigh the value of the cow and her production. Emotionally is can be hard too - you buy in this lovely old cow and 2-3 years later she's had it.
My advice would be to spend your hard earned dollars doing one of two things - or both. Buy a calf and rear her yourself. This way your going to get a friend who does you well for the rest of her life. Sure, it means waiting 2 years before you can milk her but believe me, the waiting is worth it. Or, buy in an in-calf heifer and spend time with her so that by the time she calves the pair of you have a rapport and again, she will do you well for a long time to come.