Welcome Tkhatun.
That´s a good observation. It is not common for birds to have their feathers dirty. They have a gland that provide oil to be spread in the feathers. That oil will keep the feathers waterproof and will give the bird´s feather a smooth and clean appearance.
Thanks for the pictures, they helped a lot. If you can observe closely, it seems to me that both birds have their feathers dirty as a whole and not just around the cheeks. I will ask you the same question fiddlejen asked you. What are you feeding them, or have they access to chew something dirty? If it is just a matter of dirty feathers you can provide a bowl with freshwater for them to clean up and the stain will be cleaned.
On the other hand some information you gave let me concerned. No, your birds are not that young. When the blue bird (which is a female dominant pied clear flight mutation) was 10 weeks old she had the head stripes all the way to the top of her nose. Her nose would not be that brown, which is in the shade of a female ready to mate (chocolate brown). They reach sexual maturity near 8 month to one year of life and your blue female has a beak/nose aspect of a female more than 2 years old. The white bird, seems to me to be a male although his mutation (pied recessive) is hard to develop the expected blue cere of regular male budgies. He is at least one year old.
I would add another level of concerning, the feathers aspect of the female (and in some degree of the white bird too) is of a rough texture. This, associated with the flake off of her beak suggest a liver overcharged; possibly due to a only seeds diet for too long and maybe little to none exposure to sunlight. Looking to the pictures you provided, they don´t seem to be moulting at all.
I recommend you to take both to the vet. Until then, switch their food for mostly pellets and some seeds with vegetables in the beginning and later only pellets and vegetables to help the liver. Expose them to early morning sunlight for 15 minutes every day. Also try to identify where they are getting their cheeks dirty. Could it be seeds dirt from the seeds bowl?
Cheers