(Reuters) - MediaBrix, a fast-growing startup whose software delivers "moment-based" mobile advertising, is going after one of the most difficult demographics in the ad business - teens.<div><a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=yhV25bSSs94:CuadmgEbxd0:yIl2AUoC8zA"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?d=yIl2AUoC8zA" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=yhV25bSSs94:CuadmgEbxd0:V_sGLiPBpWU"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=yhV25bSSs94:CuadmgEbxd0:V_sGLiPBpWU" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=yhV25bSSs94:CuadmgEbxd0:-BTjWOF_DHI"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=yhV25bSSs94:CuadmgEbxd0:-BTjWOF_DHI" border="0" /></a></div><img src="http://feeds.feedburner.com/~r/reuters/technologyNews/~4/yhV25bSSs94" height="1" width="1" alt="" />