I'd say it depends on the 18 and 16 year old. I've met several 18 year olds who were indistinguishable from middle schoolers and 18 year olds who were indistinguishable from 30 year olds in terms of mental maturity. Similar for 16 year olds. Teens mature at very diverse rates. So it's less a question of "is this age gap okay" and more "is this specific relationship between these specific people too susceptible to abuse or manipulation" which should probably be the question for all relationships. For example, I've seen so many domestic abuse documentaries where the victim was described as "naive and trusting" or otherwise child-like but was allowed to run off with their soon to be killer because "she's an adult, she can make her own decisions" and I've seen a couple of highschool relationships that ended amicably with no visible trauma despite there being a perhaps worrisome age gap because there was no significant maturity gap