When I was in 5th standard I was very smart aswell.. always used to top in the class.. but now 15 years down the line I have forgotton some stuff.
It is same for everyone. I am dead sure that 15-20 years down the line when these 5 "classmates" will have settled down with highly qualified degrees and stuff they will not be smarter than 5th Standards.
And judging by the questions depicted in the show, I really doubt it that they have come from text books of upto 5th Standard... come on yar.. afterall 5th standard students are only 9-10 year old kids and u cant expect them to know all that.