You are right. To make a 2.30 hrs + movie engaging without songs, action, romance, comedies, a very commendable job indeed. But I would have loved to see an older version of Iyer.
Let me try and figure out Iyer's age
In CBI Diarykurippu, he was shown talking to his son over telephone while Urvashi and Bahadur is in the office, from the nature of conversation I am guessing the son is 5-6 years old. So Iyer must be around 32-33 I suppose? And Devadas seems to be in the same age group of Iyer more or less.
In Sethuramaiyer CBI, we see Sathyadas, the son of Devadas is already a DySP, so i am assuming he is atleast 30? So if Iyer and Devadas had sons with similar age then Iyer was around or over 55 in that movie. But they did not make him look that age. I am assuming CBI 5 happened after 8-9 years after Sethuramaiyer CBI since Renji Panicker was narrating the plot that happened in 2011 or 12 if I remember correctly. So Iyer should be actually a 63-64 year old in CBI 5.