I am under the impression that the old ones get less fidgety when I am around. Guess they know the tricks of the trade and realise no harm is coming to them. And yet the eye remains alert even if their head is turned some angle away from you.
When hungry and busy fishing, young or old are often so tense they care less about what is happening further away as long as you move gently. Especially if they stand in flowing water. Which by the way can be an asset for us as the water noise covers the rustle we sometimes make when moving around in tall grasses and such like.
When basking in the sun after a nippy night and early morning they also seem more willing to accept you at close quarters. When their comfort zone is being threatened they stretch their head out and start looking nervously right and left and have got you framed. Looking for possible escape routes? When they start bending a little forward and you sense that slight flexing of the legs, take off is about to happen. then you clearly disturbed the bird, but I notice some people are deliberately moving towards the heron to get a BIF shot that way.