Here in the western U.S., mares will come into season (estrus) during the warmer months, say April to maybe October. I have friends in some southern States that say their mares will come into season year round. Their cycle is usually lasts about three weeks and a noticeable change is displayed in your mare. If not around a stud or even a gelding, you might notice a holding of the tail higher after a very brief urination. She will tend to squat, as well. If she's around a stud or even an interested gelding, she'll be quite interested, which she would not be if not in season. Here in horse country, the best way to determine her season is to put her in proximity to a tethered stud. She'll be very interested, often nuzzling or "presenting" if she's deep in heat... By the way, for a healthy, active mare, 15 years old isn't too old by any means...