I've never seen any evidence stem cells (other than embryonic stem cells) can re-elongate their telomeres. At best some residual telomerase activity can slow the rate of loss.
Adult stem cells express telomerase, and telomerase elongates telomeres. How much it does this is stochastic: some may shorten and some may lengthen, depending on factors like the amount of telomerase, the antioxidant status, etc. But there is an optimum--
"This work shows that the optimal length for telomeres is a carefully regulated range between two extremes," says Jan Karlseder, a professor in Salk's Molecular and Cell Biology Laboratory and senior author of the work. "It was known that very short telomeres cause harm to a cell. But what was totally unexpected was our finding that damage also occurs when telomeres are very long."
...As telomeres shorten over time, the chromosomes themselves become vulnerable to damage. Eventually the cells die. The exception is stem cells, which use telomerase to rebuild their telomeres, allowing them to retain their ability to divide, and to develop ("differentiate") into virtually any cell type for the specific tissue or organ...
Perhaps not surprisingly, cells with too little telomerase had very short telomeres and eventually the cells died. Conversely, cells with augmented levels of telomerase had very long telomeres. But instead of these cells thriving, their telomeres developed instabilities.
https://medicalxpres...y-telomere.html
In fact, stem cells trim back telomeres if they get too long. From the same article--
The team observed that very long telomeres activated trimming mechanisms controlled by a pair of proteins called XRCC3 and Nbs1. The lab's experiments show that reduced expression of these proteins in ESCs prevented telomere trimming, confirming that XRCC3 and Nbs1 are indeed responsible for that task.
