20 years isn’t long for ancient writings. A lot of history we have from that time is written hundreds of years after the fact, while this was merely decades. And it’s pretty consistent. For legends to form typically, generations had to pass, yet the earliest records show Christians always believing that Jesus rose from the dead, leaving little room for speculation or evolution. Contrast this to Marian dogmas where things get added on like the perpetual virginity, her being sinless, assumption of Mary, etc, which are small details developing over centuries. Or St Patrick and the snakes. While the Gospel accounts just pop up at around the same time close to Jesus’s life on earth.

Secondly - that 35 year number is flawed. Averages are calculated by adding up the lifespans and dividing the total number by amount of statistics. People weren’t dying at 35. Instead, there was a high infant mortality rate. Caesar was assassinated at 55 years old, Tacitus died at 64. Gordian I lived to be 79, Celsus to 75.