It's solely dependent on the person and their experiences. I know I personally aged a lot from 16 to 18, but some people mature a little faster, some a little slower. Some not at all.

I think 18 is a decent starting cap, but for some people 16 is their 18, and for some it's their 14 or even 13. So it's hard to say.

The manipulation comment is based on my own experiences with teens I've known, so it's subject to bias.