1) But it wont't. I know plenty of people who started having sex before being even 18, and I know aproximately zero people who have had any lasting ill effects because of it.
2) Yes and no. While we do live in a culture where children are generally treated incredibly lightly and often over-nurtured or overprotected, it doesn't always lead to mental maturity coming later. I know many people who are probably slower to mature, but I also know many who reached mental maturity for before 21. Now, this perhaps might be affected by the fact that I live in a rural community where it is not uncommon to start work at 14, but still...
Just to kind of explain my perspective on this, where I come from, the majority of people from 14-25 spend a great deal of time in various stages of inebriation. For most people, there is nothing else to do. It's literally "what do we do this weekend?" "party at X place, invite the whole school and everybody's brothers/sisters/cousins/friends". I have yet to see significant negative results coming out of this.
Now, I know these are just anecdotes, but since I have never really seen any hard fact based arguments against these things...