Im pretty sure that I have heard that about seats before. That they have a useby date which is 4yrs after the date they were made.

On the 5yr old... The new law states thata child between the age of 4 & 7 MUST be in a booster seat. I serioulsy doubt that a 6 yr old can "legally go into a adult seat with these new laws. So I wouldn't be relying on that print out. Besides that, My 7 yr old can legally go into a adult seat but there is no way in hell he will be.

You need to make sure that when they are seated in the adult seat that 1 their bum is positioned right back. 2 that their knees are bent at the edge of the seat & 3 their feet sit on the floor.
Also the seatbelt needs to be positioned over their pelvis NOT across their stomach. My 7 yr old can't sit with out his booster as the seatbelt would sit on his stomach. Not only that but the buckle part sits over his stomach also. Can you just imagine the damage that would do to his insides in the event of an acciendent?

Legal or not.. You really have to look at how the seatbelt & child is positioned before thinking its ok because its "legal". I am pretty horrified when I see kids smaller then my 7 yr old sitting in a adult seat with nothing to bring them to the right positioning.
Regardless of age, kids vary so much in size. A 7 yr old is NOT an adult & therfor is not suited to being in an adult seat/seatbelt.