Every 16 yr old girl you(and I) know is a nida, so bringing a proof from how we view things is a bit tough.
Dovid hamelech was gozer on Yichud with a penuyah. You tell me, hwas there any gezera ever about touching a penuya.
The torah discusses an issur of kereiva to arayos. Who extended it to non arayos(16 yr old non nidas and 3 yr old girls)?
There is a concept of not coming close to an aveira (asu seyag latorah) so lewd speech or behavior might get included as a seyag to Lo tihyeh kadeish, but that would hardly be relevent to giving a 3 yr old niece a kiss or a pat on the back(whereas it might be relevent to your 16 yr old unmarried neighbor who you know goes to the mikva)
RH: Why should it be different between an 8 year old and a 16 year old (who went to the mikva)? Are you suggesting that it is okay for anyone to touch the 16 year old girl?