The usual theory of parent/teenager conflict has it that the teenager is testing boundaries, seeking autonomy, differentiating from the family unit, full of hormones, acting out. It’s the teenager who has changed.
That’s obvious. The child is changing into an adult. Both parent and child are new at this.
In his My Lives Edmund White puts more of the onus on the parents:
“Through friends my age I have noticed that as teenage boys and girls become more and more beautiful their parents are reduced to continual spluttering rages against them. It’s enough for a slim-waisted, broad-shouldered lad to come skateboarding up to the curb with a flick of his full black hair and two dark red roses surfacing in his white cheeks to make his balding, stooped father yelp with hate-heavy vituperation. I suppose hate is the only way to divert criminal desire – or an equally disagreeable acknowledgment of envy and spite.”