Why does it become harder to concentrate as someone gets older?

probably reading more boring stuff. also increased self awareness. all the feelings and self-judgments are distracting

but there's things i can still get absorbed in for ages - like anything to do with solving puzzles or codes or maths or something, or fixing things, or reading good fiction. some academic stuff doesn't hold my attention at all tho cos it's like there's no work for your brain to do, you're just reading someone's point of view