One line of science kits that lasts a long time and can be used by a wide variety of ages is Snap Circuits. We bought our kit a couple of years ago and the kids still play with it all the time.
All the parts are mounted on plastic bases that simply snap together to form a circuit. The one pictured is the mid-priced 300 part set; they also make 100 and 500 part kits. Even when Ds#3 was only 4 years old he was snapping these together and making circuits.
And on birthdays Ds#2 likes to put this together: