A Vet can usually tell the age of a kitten, very young ones at least, by the teeth. Back teeth, as with children, come through later than the front ones.

Also kittens under 6 weeks will have blue eyes. In moggies, that starts to change to green at 6 weeks.

Personally I dont like using Supermarket kitten milks, I much prefer the Royal Canin Baby Milk, Cimicat, or any of those, which I know to contain all that is needed. BUT... I dont use them when a kitten is over 6 weeks and weaned.

Water and good food is far better..

UPDATE

I have just looked at Mabels photos.. she most certainly on those photos is 7 - 8 weeks. She may have lost weight due to her illness, hence she may have resembled a 4 week old kitten, in weight.