Whilst there are always exceptions to any rule, we suggest that any of the kits on our site are unsuitable for any child under the age of 7 or 8 years of age. This is due to the small size and fiddliness of the parts and the level of concentration required.
With close supervision I'm sure your bright youngster could accomplish the easier kits, but you'd also stand a good chance of putting them off for life...
In the best traditions of nanny-state-ism, I think I have to say;
'Unsuitable for children under the age of 36 months. Contains Small Parts.'
At the other end of the spectrum, please be aware, again, that the parts are small and may be difficult to put together without good light and a decent magnifying system if you are far-sighted.