This claim is entirely untrue.
Although it may be rare, once the female body goes through ovulation nearly 2 weeks before the first period, pregnancy is possible. https://www.yourfertility.org.au/everyone/timing According to BBC News https://www.bbc.com/news/magazine-34195973 in the year 2015, a 10 year old victim in Paraguay arrived at the hospital 20 weeks pregnant. We see in the same article more than 700 girls younger than 14 years old also gave birth. On average women begin to ovulate between the ages of 12-14, but it can happen to girls as young as 8. https://www.plannedparenthood.org/blog/can-you-get-pregnant-at-any-age
That being said, pregnancy at a young age could be life threatening due to narrow pelvises. It can cause obstructed labor, and leads to risk of pelvic inflammatory disease.
Just because pregnancy at a younger age is rare, does not mean it is impossible. This claim is untrue.