Late 30s/early 40s is definitely my sweet spot. These chicks hang around a lot longer and are certainly more fun. I will date the occasional chick in their 20s, but that never really lasts very long... To them I am just their older man 'experience', but it is fun watching them when I don't chase them like an idiot. They actually get ticked off, but quickly move on to some fool who will play her games.
Right now I'm regularly dating two, both in their late 30s, and one that I just started seeing that just turned 30. But I have a couple of FWB chicks in their early 40s. The sad truth is that as I get older, it becomes really hard dating multiple chicks. Ten years ago, after my divorce, I was really crushing it. I find it almost impossible to causally date more than a couple at a time... I might look like I'm in my early 40s, but inside I am fully in my 50s.