I have a 28 yr old gelding, and he urinates more frequently than he used to as well. He started it about 6 years ago, when he would urinate right before he got fed, no matter what. I ran blood, he was fine. And he continued to do that--in addition to whatever he did in the stall/pasture.
He has an automatic feeder now, which feeds him small amounts 12 times a day--so he urinates when it goes off. He also urinates when he sees me or my husband coming out to visit. So we're talking about, like 12-14 times a day when I know he is urinating, THATS ALOT!!!! It has become a habit I think. It always looks normal. And he had bloodwork/UA done last month which came back completely normal. Sometimes he even stretches out and nothing happens. He has always had the cleanest of sheaths because of this. So the only thing I can think of is habit, and he urinates whenever he sees/hears a "trigger". I'm going to get fed, time to stretch out...
Not sure if that helps, but that is my experience. Just start ruling causes out.