What age can a child use a booster seat instead of a full child seat?

What age can a child use a booster seat instead of a full child seat

Re: Booster seat

This article seems to answer your question.

The VHI gives [broken link removed] advice but it seems to be more suited to America.

As sues articles say its done by weight rather then age. I think 40lbs (discovered recently while trying to rent a car with the seats) is the recommended starting weight for a booster however our 4 yr old recently got too big (tallish) for her child seat so we've put her in a booster but shes only 34lbs. Shes grand in it and both myself and missus car are happy with it.
Conversely, her younger sister is just gone 3, is a bit of a chunky monkey so is actually the same weight but we wouldnt be happy with her in the booster yet.
Id say get one and try the child in it to see yourselves, I mean, youre going to have get one at some stage. The one drawback I find with them is that theyre uncomfortable for the child to sleep in as they cant be arched back which may be factor in deciding which one to use on longer journeys.

Who quotes weight as determining suitability for a booster seat. Height is what determines whether a child should be moving to a booster seat. A short, fat child might weigh 40 lbs, and be entirely unsuitable for a booster seat. A tall, skinny child can come in at well under 40 lbs, and be suited to a booster seat. It's essentially to ensure that the seatbelt isn't sitting too high and going across the child's throat.