[d3.js] d3.select("#element") not working when code above the html element

Use jQuery $(document) function...

$(document).ready(function(){

var margin = {top: 20, right: 20, bottom: 30, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x0 = d3.scale.ordinal()
    .rangeRoundBands([0, width], .1);

var x1 = d3.scale.ordinal();

var y = d3.scale.linear()
    .range([height, 0]);

var color = d3.scale.ordinal()
    .range(["#98abc5", "#8a89a6", "#7b6888", "#6b486b", "#a05d56", "#d0743c", "#ff8c00"]);

var xAxis = d3.svg.axis()
    .scale(x0)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")
    .tickFormat(d3.format(".2s"));

//d3.select('#chart svg')
//d3.select("body").append("svg")


    //var svg = d3.select("#chart").append("svg:svg");

    var svg = d3.select("#BarChart").append("svg:svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
    .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

    var updateData = function(getData){

    d3.selectAll('svg > g > *').remove();

    d3.csv(getData, function(error, data) {
      if (error) throw error;

      var ageNames = d3.keys(data[0]).filter(function(key) { return key !== "State"; });

      data.forEach(function(d) {
        d.ages = ageNames.map(function(name) { return {name: name, value: +d[name]}; });
      });

      x0.domain(data.map(function(d) { return d.State; }));
      x1.domain(ageNames).rangeRoundBands([0, x0.rangeBand()]);
      y.domain([0, d3.max(data, function(d) { return d3.max(d.ages, function(d) { return d.value; }); })]);

      svg.append("g")
          .attr("class", "x axis")
          .attr("transform", "translate(0," + height + ")")
          .call(xAxis);

      svg.append("g")
          .attr("class", "y axis")
          .call(yAxis)
        .append("text")
          .attr("transform", "rotate(-90)")
          .attr("y", 6)
          .attr("dy", ".71em")
          .style("text-anchor", "end")
          .text("Population");

      var state = svg.selectAll(".state")
          .data(data)
        .enter().append("g")
          .attr("class", "state")
          .attr("transform", function(d) { return "translate(" + x0(d.State) + ",0)"; });

      state.selectAll("rect")
          .data(function(d) { return d.ages; })
        .enter().append("rect")
          .attr("width", x1.rangeBand())
          .attr("x", function(d) { return x1(d.name); })
          .attr("y", function(d) { return y(d.value); })
          .attr("height", function(d) { return height - y(d.value); })
          .style("fill", function(d) { return color(d.name); });

      var legend = svg.selectAll(".legend")
          .data(ageNames.slice().reverse())
        .enter().append("g")
          .attr("class", "legend")
          .attr("transform", function(d, i) { return "translate(0," + i * 20 + ")"; });

      legend.append("rect")
          .attr("x", width - 18)
          .attr("width", 18)
          .attr("height", 18)
          .style("fill", color);

      legend.append("text")
          .attr("x", width - 24)
          .attr("y", 9)
          .attr("dy", ".35em")
          .style("text-anchor", "end")
          .text(function(d) { return d; });

    });

}

updateData('data1.csv');

});