You have every right to be angry and upset about this. Seven or eight year old children are not 'activated a few years early' by a much older step brother. The correct terminology for it is sexual abuse.
In the course of normal childhood development seven and eight year old children are not interested in being sexually active. I don't know what your t was thinking.
I was not 'activated early' by my somewhat older brother. I was sexually abused by him. Neither was I 'activated early' by my father. He too sexually abused me. And the others. They did not 'activate me sexually'. It was all abuse of an available and vulnerable child for their own sexual purposes.
Your T was straight out wrong to say what he said.
He may well be a great T in other ways, but in this particular instance he made a very inappropriate and potentially very damaging comment to you.

And,
it makes me seriously question why he has a need to normalize that kind of behavior.

By the way, the brother who 'sexually activated' me was also 'sexually activated' by our father.
Meh. So it 'activated' us a little early.
No problem, right?