Why is it said that a person's social class is basically fixed by age 35?

Class fixation isn't about being 35,

but depends on what age you have children. As long as you have children, without overwhelming wealth, no matter how old you are, your social class is set.

Otherwise, do you think that's why Seris keeps urging you to have children?

Without the burden of family descendants, you'll find that your life has unlimited possibilities and energy. You want to learn new skills, broaden your horizons,

spontaneous travel, breaking through class barriers—if you dare to think and have the ability, you'll realize that life isn't a track, but a wilderness.