Turning your data into a pivot table would be the way I would do it. Add store and postcode to the rows, and add age to values and change the field value settings to average. But this formula will do it too.
<style type="text/css">table.LEGO-table {font-size: 12px;border: 1px solid #CCC;font-family: Arial, Helvetica, sans-serif;border-collapse: collapse;}.LEGO-table td {padding: 4px;margin: 3px;border: 0.1px solid #000000;}.LEGO-table th {background: linear-gradient(0deg, rgba(9,121,22,1) 24%, rgba(51,51,51,1) 100%);color: #FFF;font-weight: bold;border-collapse: separate;border: 0.1px solid #000000;}td.blank {background-color: #ebebeb;text-align: center;}.LEGO-table tr:nth-child(even) {background-color: #d9d9d9;}</style><table class="LEGO-table"><caption>LROBBO HTML</caption><thead><tr class="LEGO-firstrow"><tr><td class="blank"></td><td class="blank">A</td><td class="blank">B</td><td class="blank">C</td><td class="blank">D</td><td class="blank">E</td></tr><tr><td class="blank">1</td><th>Store</th><th>Postcode</th><th>Age</th><th></th><th></th></thead><tbody><tr><td class="blank">2</td><td>store1</td><td>1234</td><td>20</td><td></td><td>36.5</td></tr><tr><td class="blank">3</td><td>store1</td><td>1234</td><td>53</td><td></td><td></td></tr><tr><td class="blank">4</td><td>store1</td><td>1236</td><td>18</td><td></td><td></td></tr><tr><td class="blank">5</td><td>store2</td><td>4556</td><td>21</td><td></td><td></td></tr><tr><td class="blank">6</td><td>store2</td><td>4557</td><td>25</td><td></td><td></td></tr><tr><td class="blank">7</td><td>store2</td><td>4558</td><td>30</td><td></td><td></td></tr><tr><td class="blank">8</td><td>store3</td><td>6771</td><td>41</td><td></td><td></td></tr><tr><td class="blank">9</td><td>store3</td><td>6772</td><td>25</td><td></td><td></td></tr><tr><td class="blank">10</td><td>store3</td><td>6772</td><td>39</td><td></td><td></td></tr></tbody></table></br></br><table style="font-family:Arial; font-size:10pt; border-style: groove ;border-color:#00ff00; color:#000000; "><tbody><tr><td><b>Formulas - Array formulas require Ctrl+Shift+Enter</b></td></tr><tr><td><table class="LEGO-table"><tr><td>E2</td><td>=AVERAGEIFS($C$2:$C$10,$A$2:$A$10,"store1",$B$2:$B$10,1234)</td></tr></table></tbody></table>