1. Science is too complicated
Whilst I think teaching advanced quantum mechanics to 3 year olds might be taking things a little too far, science is all around us and in everything we see and do. Anything is complicated when you look at it in enough detail.
Teaching the basic principles of science including: experimentation, observation, deducing / concluding findings, evidence based approaches, discovery, analysing, questioning, finding out the answers are things that children do naturally.
We need to nurture this in built curiosity and start to hone these skills from an early age. Skills that will be useful in all walks of life whether or not you end up becoming the next Stephen Hawking.