it's not the age group that appeals, it's the individual woman.
but if forced to generalize I'd say women between 30 and 40 are most likely to have the best combination of ingredients
A lot of women younger than me (I'm 31) tend to get on my nerves. When you find one who is cool and mellow though, she's a gem.
Even back before the whole "MILF" thing (oh where would we be, if it weren't for the American Pie movies? :1orglaugh ) I was into older women though.
Most women I like fall into the age group of 25-35, though there are many women I find quite attractive both above and below that range. As others said, it depends on the indivudual woman.