Can asthma develop with age, as in 27 years old and just now showing symptoms? And is Benadryl (diphenhydramine) bad to take if showing symptoms of asthma?

Any age. Asthma can develop at any age. Most children who develop asthma will have symptoms before 3 years old. 20% of adult onset asthma will have an occupational trigger for their asthma. Old information suggested Benadryl (diphenhydramine) and old antihistamines could worsen asthma. This is not the case, but the newer antihistamines such as claritin, zyrtec, and Allegra are effective and do have less side effects.