Testing Experimental Medication in Healthy Subjects

Eligibility Details: Inclusion Criteria: 1. Be normotensive, with no history of hypertension or use of antihypertensive medication. Blood pressure (BP) must be less than (<) 140 millimeter of mercury (mmHg) (systolic) and <90 mmHg (diastolic). Healthy Adult and Elderly Participants (Parts A through D and Part F) 2. Must have a body mass index (BMI) greater than or equal to (>=) 18.0 and less than or equal to (<=) 30.0 kilogram per square meter (kg/m^2) at the screening visit (non-Japanese only). Healthy Adult Participants (Parts A, B, C, and F) 3. Must be aged 18 to 55 years, inclusive, at the screening visit. 4. Must have a body weight >=50 kilogram (kg) at the screening visit. HE Participants (Part D) 5. Must be aged >=65 years, inclusive, at the time of informed consent. 6. Must have a body weight >=40 kg at the screening visit. Healthy Japanese Adult Participants (Part E) 7. Must be aged 18 to 55 years, inclusive, at the screening visit. 8. Must have a BMI >=18.0 and <=26.0 kg/m^2 at the screening visit. 9. Must have been born in Japan to a Japanese mother and father and have maternal and paternal Japanese grandparents. 10. Must have not been away from Japan for more than 10 years at the screening visit. 11. In the opinion of the investigator, must have a lifestyle that has not changed significantly since relocation from Japan. Exclusion Criteria: 1. Has a known hypersensitivity to any component of the formulation of TAK-994 or related compounds. 2. Has a risk of suicide according to endorsement of Item 4 or 5 of the screening/baseline visit Columbia Suicide Severity Rating Scale (C-SSRS) or has made a suicide attempt in the previous 6 months. 3. Has a lifetime history of major psychiatric disorder, such as bipolar disorder or schizophrenia. Participant who have history of major depressive disorder (MDD) may be included, but participants who have current active MDD or who have had active MDD in the past 6 months are excluded. 4. Has a clinically significant history of head injury or head trauma. 5. Has a history of cerebral ischemia, transient ischemic attack, intracranial aneurysm, or arteriovenous malformation. 6. Screening electrocardiogram (ECG) reveals a QT interval with the Fridericia's correction method (QTcF) greater than (>) 450 millisecond (ms) (men) or >470 ms (women). 7. Has a resting heart rate outside of the range of 45 to 100 beats per minute, confirmed on repeat testing within a maximum of 30 minutes). Healthy Non-Japanese Adult Participants (Part C and Part F) 8. Has undergone CSF collection within 30 days before check-in (Day -2 [Part C] or Day -1 [Part F]). 9. Has a known hypersensitivity to anesthesia or its derivatives used during CSF collection or to any medication used to prepare the area of lumbar puncture.