Telomerase is an enzyme that regulates the length of telomeres, which are repetitive sequences of DNA at the ends of chromosomes that protect them from damage and deterioration. Telomerase is essential for cells to replicate indefinitely, such as in embryonic development and stem cells. However, most human cells switch off telomerase after birth, except in some tissues such as the testes and ovaries.

According to some sources¹², human embryos have high levels of telomerase activity in the blastocyst stage, which is about five days after conception. Telomerase activity remains high in fetal tissues until about sixteen to twenty weeks after fertilization. Then, it rapidly declines and becomes undetectable after the neonatal period, which is the first twenty-eight days of a child's life post-birth.

Therefore, based on these sources, the human embryo switches off telomerase sometime between sixteen to twenty-eight days after birth. However, this may vary depending on the tissue type and individual factors.

I hope this answers your question in a better, precise, and accurate way. If you need more information or clarification on this topic, you can check out some of these resources:

- ¹ Telomerase in Human Development by The Embryo Project Encyclopedia
- ² Telomerase in Human Development by Arizona State University
- ³ Fetal Development: The 1st Trimester by Mayo Clinic
- ⁴ Scientists Discover an On/Off Switch for Aging Cells by Salk Institute

Good luck with your model!👍

Source: Conversation with Bing, 4/15/2023(1) Telomerase in Human Development | The Embryo Project Encyclopedia. https://embryo.asu.edu/pages/telomerase-human-development Accessed 4/15/2023.
(2) Telomerase in Human Development - Arizona State University. https://embryo.asu.edu/printpdf/pages/telomerase-human-development Accessed 4/15/2023.
(3) Fetal development: The 1st trimester - Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302 Accessed 4/15/2023.
(4) Scientists discover an on/off switch for aging cells. https://www.salk.edu/news-release/scientists-discover-an-onoff-switch-for-aging-cells/ Accessed 4/15/2023.
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