No this is not normal senescence or normal fade.
You either started a flush way to early or have some other issues but this late in flower just hope you don’t have botrytis or other serious issues and get her to her chop date
This can happen at the end of its life (which it is) You can either pluck em or wait till harvest up to you. Less to do come harvest if you pluck the yellow fellas now.
depends on how you finish. If you flush then yes this can be normal. If you don't flush because it is a myth and properly feed then no its not normal. Need more details on how you are growing.