What is the most common source of SSI?

Could you please elaborate on the most prevalent source of SSI, or Supplemental Security Income? I'm particularly interested in understanding what constitutes the primary means through which individuals and families access this vital financial support program. Is it primarily based on age-related eligibility, disability status, or a combination of factors? Furthermore, are there any recent trends or changes in the distribution of SSI sources that might shed light on the evolving needs of the population it serves?