Equine dentistry is becoming better understood in the past 5 years. I would almost bet money that your 15 year old needs its teeth done. They don't always drop grain. Also over time if their teeth aren't done they can develope some pretty nasty sores and callesus in their mouths. Also as a horse gets into its later years their tooth eruption slows done and they may need their teeth done less. But in general a horse under 5 years should have their teeth checked every 6 months and floated yearly. After the age of 5 years checked yearly and done every other year. Some need it more or less depending on their mouth (deformities) conformation, their diet, etc. Also by keeping up on their dentals you can avoid a number of behavioral problems. I to in the begining was told that only older horses needed their teeth done, but I know better now and would never send a youngster off for training (2years) without have a dental done first. If you ever get the chance to look in the mouth of a young horse before a float and have the oppurtunity to feel in there it is amazing how sharp their teeth can be. I have cut my finger a time or two on a young horses sharp mouth.

Hope this helps.