There are heaps of 18 year olds playing since round 1, eg SPP, Logue, McGrath, etc. It depends on the player and his physical maturity. OK some get injured, eg Ainsworth, but so do older players.
Pavlich debuted early on and his birthday is not until the last day of the year meaning he was many months younger than some others in the same draft. But of course he was 193cm and 90kg. So it's ironic he wants the age lifted because he would never had played 350+ games and be employed for 3 years out of school.