Who performs a diagnostic study when trying to certified someone over the age of 18 for a 1st degree felony committed when they were 14?

I have a 20 year old who committed Aggravated Sexual Assault of a Child when he was 14. Victim didn't outcry until respondent had aged out. I've researched §54.02(j) and the diagnostic study is the only thing I'm not sure about. First time that I've had a contested certification, any help would be appreciated.