Final answer:
AIDS is diagnosed when the CD4 T-cell count falls below 200 cells/\u00B5L, indicating the progression from HIV to AIDS in an individual.
Step-by-step explanation:
A 45-year-old HIV-positive individual may progress to AIDS when their CD4 T-cell count falls below a specific level. Based on the information provided, the development of AIDS is indicated by a CD4 T-cell count that is < 200 cells/\u00B5L of blood. A CD4 T-cell count falling beneath this threshold is a clinical marker that signifies the transition from HIV to AIDS, which involves a severely damaged immune system and increased vulnerability to opportunistic infections and certain rare cancers.