Read a theory about One/Henry/Vecna being Eleven’s dad before…
The theory claim that Brenner wanted to create a child with stronger powers, so he waited until One was old enough to become a father.
The hosts explain, "The theory is that Brenner knew that none of the kids, numbers Two through Ten, were as strong as number One. So, Dr. Brenner waited for One to get older to make him breed with Terry Ives to create Eleven
Eleven's mum, Terry Ives, joined the MKUltra experiments at Hawkins Lab in 1969. Eleven was then born in 1971, which suggests that her mother became pregnant during the experiments, around 1970. Then, the theory takes us back to 1959, when One (aka Vecna) was shown to be 12 years old - making him around 24 in 1971 when Eleven was born