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?