Why does temporal arthritis mainly only happen is older ppl? why isn't it coomon to happen in a 20yo?

Old arteries: This is speculation, but aging and degeneration of arteries may expose parts of the arterial wall and this triggers an immune response. This doesn't happen with young healthy arteries.

You're too young: Temporal arteritis mainly affects people over 70 as they get the diseases that cause it. Young people don't. Unlikely under 50. It causes arteries to become blocked & can lead to vision loss & stroke. Diagnosis is made by blood tests & a biopsy of the temporal artery. Symptoms: headache, scalp tenderness (when brushing hair or touching their temples) , pain with chewing, fever & muscle weakness.