Stem cells are the cells that repair your body. Their potential repair power comes from their free and nonspecialized state, which allows them to become any type of cell in your body. For example, a free stem cell could become a brain, heart or muscle cell as needed.
To keep your pool of stem cells plentiful, stem cells must also divide to make more free stem cells. The number of times a free stem cell can divide to make more free stem cells, is called self-renewal. As free stem cells age though, their ability to self-renew declines rapidly.[1]

Self-renewal estimates of human stem cells at various ages:

Human Stem Cell Age Self-Renewal Estimate
Placental or Umbilical 2,000,000
Teenager 200,000
40-year-old 1,000
80-year-old 4
This is the reason younger people are preferred as bone marrow transplant donors for cancer treatments. Most donor programs do not allow donors past age 60.[2] Thus, not all stem cells are equal. A stem cell’s age is one of the most significant things for determining a stem cell’s dividing and healing potential.

References

1. He, S., D. Nakada, and S.J. Morrison, Mechanisms of stem cell self-renewal. Annu Rev Cell Dev Biol, 2009. 25: p. 377-406. 2. U.S. Department of Health and Human Services. Joining the Registry Questions. How do I become a marrow donor? [Date Accessed: 1/2018]; Available from: https://bloodcell.transplant.hrsa.gov/donor/joining/joining_faqs/index.html.