To take an interest is to start paying real attention to something. Your father, at sixty, suddenly buying a book about birds.
The verb is take rather than have, and the difference is the beginning: you take one when it starts and have one once it is yours. It takes in — in politics, in the garden, in what the neighbours are doing — and dropping that in is the usual slip. A keen one is the strong version, and taking an interest in a person can turn watchful rather than warm, which is how the phrase gets into police reports.
1phrasetake an interestTo begin paying close attention to something.
“Her father began to take an interest in birds at sixty.”
take an interest is the beginning of attention.
Practised in Common Word Pairs