Daniel Radcliffe was born on July 23rd 2989, so he was 11 in 2000, when they were filming the first movie. There finishing off PoA now, in 2003, when he’s just turned 14.
So, no, he’s not 18.
It is, btw, not true that he will NOT appear as Harry in GoF. No one knows yet. Warner Brothers have denied the rumours, that Radcliffe is quitting.
For die-hard fans http://www.danradcliffe.co.uk/