When I first started playing Warhammer 40k I was told not to get an army with an old codex because they suck. So I picked Necrons because I herd they had a new codex coming out soon. Later some one told me to play against tau because they would be easy cause of the old codex. Now I have a hatred for Tau because I have had my but kicked 3 times in a row. I have also seen Grey Knight (arguably the most overpowered army) get wiped off the table by Orks.
Just because a codex is old does not mean its not good. A Old codex is harder to use however. For example Black Templar have the oldest codex in the game right now. To be successful with this army I have to understand how this army should be played. Black Templar are mainly a close combat space marine army. If i try and turn them into a shooting army I am not going to do well. In Warhammer 40k the codex doesn't suck. The way you play the codex sucks. In order to understand how your army is played you have to study your codex by reading it a lot. You can also search the internet to find out how other people play that army and use their tactics to make your own. In order to win any battle you must understand your own strengths and weaknesses and your enemies strengths and weaknesses. Using an old codex has its strengths and if used properly it can kick but. Also The most important thing is to have fun. If you don't enjoying playing as your army then you probably should play a different one weather it is a new codex or old.