Do mares stop coming into heat when they get a certain age?

I rescued a mare 2 weeks ago she's goin on 24 in march, she hasn't came into heat yet and i'm starting to worry I have a proud cut gelding so I would know if she came into heat but he acts like she's a gelding he has nothing to do with her and she kicks at him everytime he gets near her hind end. I know that it's winter but our 12 yr old mare is already in heat could she just be not coming into heat cause of winter? or could she actually be bred? I took her to a vet he said he didn't think she was bred but said if she was she would have one in 10 months. from what I've heard about this mare her 3rd foal was unexpected as she was vet checked and told she was open and she had a foal 4 months later so i'm wondering if that can actually happen and if that's happening now? I do plan on taking her for a ultrasound soon I just a few answers to maybe ease my mind some



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