In the states it varies widely. I'm in WA (obviously

), and here it is the same - 1 year and 20 lbs for forward facing, 4 years and 40lbs for a non-harnessing booster (so, a belt positioner), and 4'9" (not sure on the weight) before using a seatbelt with no booster.
But these are just guidelines. It's safest to keep your child rearfacing to the max the seat allows, then to keep them in a harnessing booster as long as possible. For a very convincing reason why, please check out
http://www.kyledavidmiller.org/ or the youtube video here:
http://www.youtube.com/watch?v=azgBhZfcqaQ
warning - you'll need tissues.