What are the main cell/molecular/etc mechanisms of human ageing that science has theory on or knows about

I'm trying to compile a broad list of the main reasons/mechanisms ageing occurs in humans. Essentially, I want to start with a broad list as a starting point so that I can get into much more detailed research about how ageing occurs and what is happening in regards to Biology at the cell and molecular level.

Google results are so broad and all over the place in regards to all this, that I really have no idea where to start and no idea what would be legitimate and reasonable to actually spend time on researching/reading about.

I will choose best answer! Thanks.

1. Telomerase activity
During each round of cell division the ends of our chromosomes (called telomers) get shorter due to the mechanisms use to replicate DNA. The enzyme telomerase can reverse this process but it is not active in all tissues so that eventually some cells cannot replicate any longer without losing genes.
(if telomerase is too active it can give rise to cancers as it immortalises the cells)

2. Mitochondria
Mitochondira produce free radicals during aerobic respiration which can damage you DNA and can make your cells age. To prevent that exercise, less food and antioxidants can help as hey reduce the amount of free radicals given off my mitochrondria (at least temprarily)

3. There is probably more stuff out there, but I am not an expert in the field of ageing.