Okay what you are describing does not sound good. You could very well be dealing with a respiratory disease. If they were all sneezing and now runny noses are circulating, it appears to be contagious and respiratory in nature. Unfortunately, you cannot just "give" them something and fix it and here are things you need to think about, consider or determine...
1. Exactly what they have. There are various types of respiratory illness and they can be either fungal, viral or bacterial. Treatment depends on what the exact problem is.
2. Antibiotics come in various medications because they treat various illnesses. They do not work for everything and one may work with fungal whereas another works with bacterial and etc.
3. What are your intentions for your birds....pets or resale?
4. Do you have access to an avian vet? Would you be willing or able to have one or all examined by a vet?
Questions I have and my opinion is:
Why are chicks are these varying ages housed together right now? Where did the chicks come from meaning did the age groups come from different sources? Are you willing to consider culling the entire flock of youngsters?
My opinion and suggestions are this....I would seek a vet exam or at a minimum, sacrifice one of the chicks showing symptoms to a State testing facility for appropriate testing to know what I was dealing with. If you are dealing with a contagious respiratory disease, culling the flock is the smart decision. Respiratory disease cannot be cured and you would need to close your flock meaning no new birds in and no birds out EVER until the flock dies off.
Research respiratory illnesses and diseases in poultry and manage the situation appropriately. I would caution you to be very wary of allowing any other birds in contact with these chicks and/or the ground they are on. Respiratory disease can spread like wildfire and it will infect an entire flock in a flash.
Also consider this...what type of bedding are they are? Is it dusty where they are? Is it drafty or windy where they are? Is the discharge from their nose colorless and/or odorless. Is their poo normal?
You need to be able to answer these questions and get to the bottom of this. Can you give more information and answer some of the questions I have asked so that I can try to supply you with some resources or further information?