Inclusion Criteria:
* Community dwelling men and women 18-35 years old or 65-89 years old
* Ability to walk unassisted for ten minutes
* Willingness to undergo all testing procedures
* English speaking Additional Inclusion Criteria for those with SCD
* Subjective report of cognitive complaints with scores \>16 on the Cognitive Change Index (CCI-20), a validated scale of subjective cognitive decline (Saykin et al., 2013). This scale consists of 20 items that are rated on 5 point Likert scale, where 1= "Normal: No change compared to 5 years ago", 3= "Mild Problem: Some change compared to 5 years ago) and 5="Severe Problem: Much worse compared to 5 years ago".)
* No evidence of dementia or mild cognitive impairment based on cognitive screening (i.e., Montreal Cognitive Assessment (MoCA) score within normal limits for age, education and sex using the NACC Uniform Data Set (UDS) norms (Weintraub et al., 2018)\[119\]
* No psychometric evidence of cognitive impairment based on performance on the Neuropsychological Battery from the NACC Unified Data Set, version 3\[119\]. The battery includes measures of attention, recent memory, language, visuospatial, and executive function. Norms are available for over 3600 older adults \[119\]. Scores on these measures cannot be lower than 1.0 SD (16th %ile) below normative values based on age, education, and gender.
* Reading at \> 8th grade level based on the reading subtest of the Wide Range Achievement Test- IV.
* Global Clinic Dementia Rating (CDR) score must be 0 \[156\]
* Family history of dementia/probable Alzheimer's disease in first degree relative (parents, children, siblings)
* Normal functional behavior in terms of daily activities, based on the Functional Activities Scale\[122\]
* In line with recommendations of SCD task force (Molienueva et al., 2017)\[116\] an informant must be available for two reasons: a) to provide information about the participant's complaints using the informant version of the CCI-20, and b) to corroborate normal IADL's on the Functional Activity Questionnaire\[120\] .
Exclusion Criteria:
* Significant medical event requiring hospitalization in the past 6 months that has the potential to contaminate data being collected (fracture, hospitalization etc.)
* Severe visual impairment or corrected visual acuity less than 20/40, which would preclude completion of the assessments
* Inability to undergo brain imaging due to claustrophobia or implants such as pacemakers, heart valves, brain aneurysm clips, orthodontics, non-removable body jewelry, or shrapnel containing ferromagnetic metal
* tDCS ineligibility for Aim 3 (history of epilepsy, medications that alter cortical excitability, etc.)
* History of stroke
* Any history of clinically diagnosed traumatic brain injury resulting in \> 1 minute loss of consciousness
* Any major ADL disability (unable to feed, dress, bath, use the toilet, or transfer)
* Report of lower extremity pain due to osteoarthritis that significantly limits mobility
* Diagnosis or treatment for rheumatoid arthritis
* Known neuromuscular disorder or overt neurological disease (e.g. Multiple Sclerosis, Rhabdomyolysis, Myasthenia Gravis, Ataxia, Apraxia, post-polio syndrome, mitochondrial myopathy, Parkinson's Disease, ALS etc.)
* Unable to communicate because of severe hearing loss or speech disorder
* Planned surgical procedure or hospitalization in the next 12 months (joint replacement, coronary artery bypass graft, etc.)
* Severe pulmonary disease, requiring the use of supplemental oxygen
* Terminal illness, as determined by a physician
* Severe cardiac disease, including NYHA Class III or IV congestive heart failure, clinically significant aortic stenosis, recent history of cardiac arrest, use of a cardiac defibrillator, or uncontrolled angina
* Is planning to move out of the area during the follow up period
* Use of walker or wheel chair
* Failure to provide informed consent
