Size 7 diapers are for kids that weigh between 35 lbs and 45 lbs and above.
Some 3 year olds are ready to wear size 7 diapers and some 4 year olds will need size 7 diapers when they are not potty trained.
Size 7 diapers can usually fit older kids who need them for both daytime as well as night time.
My 6 year old fits into size 7 Pampers Cruisers which he wears when we are out shopping or when we are on a road trip just in case we can't get him to a bathroom on time.
Sometimes when we go to stores such as Walmart and shop the bathrooms may actually be closed when he needs to use the bathroom for pee.
So he wears the diaper just in case because he has wet his pants before and I convinced him to wear diapers because it would be less embarrassing for him to wear the diaper and wet it than it would be to have wet pants.
He agreed to it and will use the toilet when he can but the diaper is there just in case and he has needed it.
At night he wets the bed so he wears those size 7 Pampers as well and he still has plenty of room to grow into the size 7 Pampers and he could probably wear them when he's 8 and even 10 years old.