I think 16 is good... But I don't believe a 16yr old should be able to have sex with a 30yr old etc. And kids underage are always going to be having sex, they're exposed to adult content earlier and earlier. Ages 14+ I think it COULD be initiated by the child and half consensual depending on the situation.
I think 18 should be the youngest for the partner to be older than 20.
It should depend on the circumstance. Whether or not the individuals were sexually active previously, how mature the individual is and how far apart the years are etc.
For instance I wasn't sexually active till I was 18, all legal age but I was still manipulated into it while drunk and didn't really want or ready for it and regretted it. So it depends on the individual. You could be manipulated at 18 even if it's legal.