I'm not sure what kind of interaction you're trying implement with the event handler, so setting that aside, I think you can simplify things if you just create a separate group for each age category.
var experiments = [
{ State_Name: "AL", Age_19_Under: 26.9, Age_19_64: 62.3, Age_65_84: 9.8, Age_85_and_Over: 0.9 },
{ State_Name: "AL", Age_19_Under: 23.5, Age_19_64: 60.3, Age_65_84: 14.5, Age_85_and_Over: 1.8 },
{ State_Name: "NW", Age_19_Under: 24.3, Age_19_64: 62.5, Age_65_84: 11.6, Age_85_and_Over: 1.6 },
{ State_Name: "NW", Age_19_Under: 24.6, Age_19_64: 63.3, Age_65_84: 10.9, Age_85_and_Over: 1.2 },
{ State_Name: "BR", Age_19_Under: 24.5, Age_19_64: 62.1, Age_65_84: 12.1, Age_85_and_Over: 1.3 },
{ State_Name: "BR", Age_19_Under: 24.7, Age_19_64: 63.2, Age_65_84: 10.0, Age_85_and_Over: 2.2 },
{ State_Name: "GH", Age_19_Under: 25.6, Age_19_64: 58.5, Age_65_84: 13.6, Age_85_and_Over: 2.4 },
{ State_Name: "GH", Age_19_Under: 24.1, Age_19_64: 61.6, Age_65_84: 12.7, Age_85_and_Over: 1.5 },
{ State_Name: "KS", Age_19_Under: 24.8, Age_19_64: 59.5, Age_65_84: 13.5, Age_85_and_Over: 2.2 },
];
var ndx = crossfilter(experiments);
var stateDim = ndx.dimension(function (d) { return d.State_Name; });
var age19UnderGroup = stateDim.group().reduceSum(function (d) { return d.Age_19_Under; });
var age19To64Group = stateDim.group().reduceSum(function (d) { return d.Age_19_64; });
var age65To84Group = stateDim.group().reduceSum(function (d) { return d.Age_65_84; });
var age85AndOverGroup = stateDim.group().reduceSum(function (d) { return d.Age_85_and_Over; });
var stackedBarChart = dc.barChart("#chart-row-Poverty1");
stackedBarChart
.dimension(stateDim)
.group(age19UnderGroup)
.x(d3.scale.ordinal())
.xUnits(dc.units.ordinal)
.stack(age19To64Group)
.stack(age65To84Group)
.stack(age85AndOverGroup)
;
dc.renderAll();
Note that if you do it this way you have to assign one of your four groups to the base chart to get things started. The three remaining charts then just stack on top of it.