The primary purpose of this study is to investigate the effects of JNJ-54175446 (dose response) on levels of Aβ fragments (Aβ1-42, Aβ1-40, Aβ1-38, Aβ1-37) in plasma and cerebrospinal fluid (CSF), the effects of JNJ-54175446 on markers of (neuro) inflammation/protection in blood and CSF and the pharmacokinetics of JNJ-54175446 followed by single and multiple dose administration.

Inclusion Criteria:

• Participants must have a body mass index (BMI) between 18 and 32 kilogram (kg)/ meter (m)^2, inclusive (BMI = weight/height^2)
• Participant must be healthy on the basis of physical examination, medical history, vital signs, and 12-lead electrocardiogram (ECG)

Part 1 and 2 (Elderly Participants):

• Healthy male or female participants between 55 and 75 years of age, inclusive
• Participant must be healthy on the basis of both physical and neurological examination performed at Screening and at admission to the clinical unit

Part 1 (Young Participants):

- Healthy male participants between 18 and 45 years of age, inclusive

Exclusion Criteria:

• Participant has a history of or current liver or renal insufficiency; significant cardiac, vascular, pulmonary, gastrointestinal, endocrine, neurologic, hematologic, rheumatologic, psychiatric, or metabolic disturbances, any inflammatory illness or any other illness that the Investigator considers should exclude the participant
• Participant has a history of hepatitis B surface antigen (HBsAg) or hepatitis C antibody (anti-HCV) positive, or other clinically active liver disease, or tests positive for HBsAg or anti-HCV at Screening (per Screening evaluations)
• Participant has a history of human immunodeficiency virus (HIV) antibody positive, or tests positive for HIV at Screening (per Screening evaluations)
• Participant has a history of malignancy within 5 years before Screening
• Participant has signs of increased intracranial pressure based on fundoscopy at Screening