I have looked in the steamapps\common\Endless Space 2\Public\Settings\WorldSettingDefinitions.xml file and it seems the star types are exactely the same for young, normal and old:

    <!-- GALAXY AGE -->
    <GalaxyGeneratorWeightTable Name="StarTypes_Normal">
        <Entry Tag="StarTypeBlue"           Weight="30"/>
        <Entry Tag="StarTypeWhite"          Weight="20"/>
        <Entry Tag="StarTypeDouble"         Weight="20"/>
        <Entry Tag="StarTypeYellow"         Weight="20"/>
        <Entry Tag="StarTypeRedSupergiant"  Weight="20"/>
        <Entry Tag="StarTypeDysonSphere"    Weight="1"/>
        <Entry Tag="StarTypeRedDwarf"       Weight="10"/>
        <Entry Tag="StarTypeWhiteDwarf"     Weight="10"/>
        <Entry Tag="StarTypeArtificial"     Weight="5"/>
        <Entry Tag="StarTypeProtostar"      Weight="30"/>
        <Entry Tag="StarTypeNone"           Weight="5"/>
    </GalaxyGeneratorWeightTable>
 
    <GalaxyGeneratorWeightTable Name="StarTypes_Old">
        <Entry Tag="StarTypeBlue"           Weight="30"/>
        <Entry Tag="StarTypeWhite"          Weight="20"/>
        <Entry Tag="StarTypeDouble"         Weight="20"/>
        <Entry Tag="StarTypeYellow"         Weight="20"/>
        <Entry Tag="StarTypeRedSupergiant"  Weight="20"/>
        <Entry Tag="StarTypeDysonSphere"    Weight="1"/>
        <Entry Tag="StarTypeRedDwarf"       Weight="10"/>
        <Entry Tag="StarTypeWhiteDwarf"     Weight="10"/>
        <Entry Tag="StarTypeArtificial"     Weight="5"/>
        <Entry Tag="StarTypeProtostar"      Weight="30"/>
        <Entry Tag="StarTypeNone"           Weight="5"/>
    </GalaxyGeneratorWeightTable>
 
    <GalaxyGeneratorWeightTable Name="StarTypes_Young">
        <Entry Tag="StarTypeBlue"           Weight="30"/>
        <Entry Tag="StarTypeWhite"          Weight="20"/>
        <Entry Tag="StarTypeDouble"         Weight="20"/>
        <Entry Tag="StarTypeYellow"         Weight="20"/>
        <Entry Tag="StarTypeRedSupergiant"  Weight="20"/>
        <Entry Tag="StarTypeDysonSphere"    Weight="1"/>
        <Entry Tag="StarTypeRedDwarf"       Weight="10"/>
        <Entry Tag="StarTypeWhiteDwarf"     Weight="10"/>
        <Entry Tag="StarTypeArtificial"     Weight="5"/>
        <Entry Tag="StarTypeProtostar"      Weight="30"/>
        <Entry Tag="StarTypeNone"           Weight="5"/>
    </GalaxyGeneratorWeightTable>

So what is the actual difference? Are young galaxies giving more steriles and old ones giving more fertiles?