This is kind of obvious, but since you are asking: you have to quit your job, quit your hobbies, delete your social media accounts, go out into the desert and spend 15 years seeking the true meaning of life.

I recently returned to a workplace I left 15 years ago. There are a substantial number of staff still there who are around my age - early middle aged. Many look very similar, but what took me by surprise - as I've never studied a large group of aging people over a decade plus - is their skin. Superficially features, hair, body weight, etc., hadn't changed but to a man (ie across everyone) they all have "aged" skin.

So, if you want to tell if someone is 25 or 40, maybe their skin will give it away?

Ignore benign random behaviors and continue to apply your agency as you see fit?

I'm sorry, my programming is limited. I'm unable to answer your question.