Yes, you did, okay, but also:
*‘It must be committed with full knowledge, both of the sin and of the gravity of the offense; "Unintentional ignorance can diminish or even remove the imputability of a grave offense.’
‘The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders mental illness.’*
So, yes, all can, but to say out and out that people do, everytime they commit a grave crime, seems to be incorrect.
The thread is about naughty teenagers, and so my point is in agreement with you in part, that teenagers can sin gravely, HOWEVER, there are going to be more not less extenuating circumstances for people who have much less life experience.