I have successfully flushed a few older cows, and have found that unless they have been flushed to death when younger, many of them flush very well. There are some younger cows, as well as older cows, that do not respond well to flushing. I have a 9 year old cow that we have got 152 grade 1 embryos out of in 6 flushes in the past 7 months. She then settled to the first AI service. That is a long ways from being a 15 year old cow, but all the same, she is older than many donor cows. As long as her health is good, and she is cycling normally, I would suggest you try her. Using older proven cows seems to make more sense to me than flushing a virgin heifer