If you've mistakenly spelled "gerastic", the correct term may be "geriatric". This refers to the medical field specializing in care for elderly individuals. Alternatively, if you meant something different, you could check if "gerastic" was an autocorrect error for "gerasticate" (meaning to gnash one's teeth) or "gerasticism" (a humorous or sarcastic expression by the elderly). Always double-check your spellings!