Show me ONE species that has an 18 year (more like 19 year, including pregnancy) waiting period.

For me, it's not the actual age so much as what it's relative to. Animals have far less to learn to reach that point where they can function as an adult of their species does in their environment. But I do agree with Dante that adolescence is a relevant marker, and I have never had a problem saying that the age of majority should be lowered to 13. I supported that even during my long Anti phase at GC. It's still somewhat arbitrary, but far less so than age 18, which is based on a human social construct, whereas 13 is closer to where nature sets the marker for childhood's end.