they all refer to a form of discrimination - they are all forms of prejudice, where someone is pre-judged on external appearance that suggests belonging to a group with perceived negative associations rather than judging that individual on their own merits.
You can discriminate against someone on the colour of their skin, the wrinkles on their face, the bumps on their chest, or anything else too - their (lack of) height, their (excess of) weight, deformity of leg, choice of editor. Anything.
And it works the opposite way too, some supposedly discriminated-against "minority" groups will support their own and discriminate against the "mainstream".
Many of the -isms do refer to this discriminatory problem, ageism as you say, but sexism and racism is just the same - its not a theory, racism means to pre-judge based on race. not all of them - feminism for example isn't one of these, its a social/political movement.