You're suppose to notice when they change, so really they shouldn't have to tell you anything.
That. If you're intimate (emotionally) with someone, at least.
You can feel the distance grow palpable, like two people driving in different directions and they get smaller and smaller.
I don't understand how anyone could argue that people don't change. Are you the same person you were 5 years ago?
I've got a moderate view of this. In healthy people, they change gradually, without losing the basis of who they were; or, you could view it like sculpture -- they always were the person they're becoming, but they were roughly hewn before and life wears away the stuff that wasn't them.
With unhealthy situations, sometimes someone suppresses/resists being themselves so much in order to survive that when they finally break down or flip, to everyone else it seems like a sudden change whereas really they just cannot maintain a charade and the other self was the lie.
There is also the stuff where someone is conscious of all this -- that on the surface they are one self, while inside feeling differently, so when they finally start acting on the inner feelings, some people think they have changed whereas really they were that way all along but are now being honest.
In any case, if you don't change with someone, you'll grow apart. Life is change, not stability.
I haven't had an experience of younger people being less malleable than older ones (I'm 42 now), my parents' generation has been infuriating painful to deal with; but then again, I've mostly been immersed in religious communities... and the older religious crowd is very entrenched, the younger crowd much more postmodern in approach. Maybe if I was in regular secular community, my experience would be different.