Yea, City defense is weak and they need to strengthen it, if they want to improve their performance to the best. The players are old and getting weak which is affecting their performance. It's very disheartening that City is be 20 points away from the top of the table. I hope that City will be able to finish on top four this season in order for them to qualify for UCL next season.
It doesn't seem like City are performing badly because of the age of Manchester City's players. We saw great performances from this squad last season too. Suddenly their poor performance this season is not due to the players in their squad. I would say guardiola has a problem with his match strategy this season . Also City's players have lost confidence. This is why the team is not able to do well against big teams. Also, selling Alvarez was one of the worst decisions by manchester city management. Alvarez scored in several important matches last season and brought City victories. If Alvarez was in the squad, City's squad would have been much stronger.