When you join APTA you also have the option of joining APTA Geriatrics, one of 18 different sections and academies of APTA.

APTA Geriatrics is your link to the resources you need to stay current in geriatric physical therapy and the best way to connect with others who share your interest in the geriatrics specialty.

APTA Geriatrics supports those therapists, assistants, and students who work with an aging population in roles of advocacy, direct patient care, consultation, supervision, and education. The Academy represents and serves over 5,500 physical therapists, physical therapist assistants, and other health care providers interested in the area of geriatric physical therapy.

There's a Special Interest Group for that.
Special Interest Groups, commonly referred to as SIGs, are communities of interest within APTA Geriatrics that can connect you with PTs, PTAs, and students who share your interests or career path. SIGs can help you grow professionally, network with thought leaders, and stay on top of the latest developments.