I disagree with Farmboy on the generic use of penicillin (or any other antibiotic for that matter) for symptoms that may or may not be indicitive of an illness. What have your temperatures been like lately? Hot days and cool nights? If so, they may simply have a virus - which will not respond to antibiotics. Are they on pasture or in a corral? If in a corral then dust could be causing the runny noses and coughing. Also, if they are in a corral and your hay is even a little bit dusty that could be causing the cough of the younger horse since they will be more sensitive to the dust than an older horse will. If on pasture, spring is in the air and pollen could be causing these symptoms as well. Since none of them are running a fever or are off feed it is likely not life threatening, but if you are concerned or if their symptoms are bothering you, then call your vet and find out what the problem is, but please do not simply administer antibiotics when you don't know what your dealing with. It will likely cause more problems than it will solve.