I bought the Oxford First Encyclopedia for my six-and-a-half year old daughter for Christmas 2005. It has been the one present that she has consistently and continually gone back to over this year, so I'm buying one for her cousin!
The wording has been well thought out and is just right for kids of around six to about ten or eleven, so the encyclopedia certainly has shelf life. Simple subjects are made interesting and difficult subjects are well explained through the use of some very stylish illustrations, photographs and diagrams.
The book has been very supportive as it covers many of the subjects taught under the national curriculum at the ages I mention above. It's also a good, solid hardback, so it has survived the school bag for nearly a year.
High recommended.