Stem cell therapy covers a wide range of approaches united by one idea: introducing cells (or the growth factors and signaling molecules those cells secrete) that can regenerate damaged tissue or replenish cell populations that decline with age. It's important to separate what's established from what's investigational here, because the field spans both. Some stem cell applications, bone marrow transplants for blood cancers being the clearest example, are decades-old standard medical care. Longevity-focused applications, using stem cells or their derivatives to broadly counter age-related tissue decline rather than treat a specific disease, are a much newer and less proven extension of that same underlying biology.
Researchers like Anthony Atala (tissue engineering and regenerative medicine) and Irving Weissman (stem cell biology) have done foundational work establishing how stem cells maintain and repair tissue throughout life, and how that capacity itself changes with age, aging stem cell populations become less able to regenerate tissue, which is part of why this field is directly relevant to longevity research even where specific anti-aging protocols remain unproven.
Compared to reprogramming or senolytics, stem cell longevity applications are more fragmented across use cases and harder to summarize with a single evidence level: a therapy validated for one tissue type or condition tells you little about whether a similar approach would work for general age-related decline elsewhere in the body. That case-by-case caution is worth applying to any specific stem cell longevity claim.