Mortal Sin and Mental Capacity

I remember reading in C.T.S. pamphlets when I was 18, and first considering becoming a Catholic, that the age when one becomes capable of mortal sin is about 7 years. Was this ever official Church teaching, or just a little idea of someone's own?

If it was official teaching, is it still?

If it never was official, is there a generally accepted age for people of average mental capacity?

The question is not loaded. Just asking.