Wondering if it's normal for the elderly to experience hallucinations when on antibiotics?

No: Any hallucinations are not normal. Older adults are more susceptible to confusion and other psychiatric issues associated with stressors. The hallucinations and confusion may be related to the infection being treated , rarely an antibiotic may cause confusion. Delirium is a frequent complication of illness in older adults.

Not unusual: Whether this is "normal" or not depends on a lot of underlying factors. One resource to understand this better is: http://www.Ted.Com/talks/oliver_sacks_what_hallucination_reveals_about_our_minds.