We picked up a redwood seedling last summer in the gift shop at the redwood forest in California. Once we got home to Chicago, we decided it would not survive outside and changed our plans to try to make it a bonsai (my 9 yr son is thrilled and very enthusiastic). It has been in a pot with regular soil since that time and has grown to about 14" with nice branch spacing. What do I do now? Should I change it to bonsai soil this Spring and at what age do I start pruning it?

Erniesmom wrote: We picked up a redwood seedling last summer in the gift shop at the redwood forest in California. Once we got home to Chicago, we decided it would not survive outside

It is a tree. It will not survive indoors either.

changed our plans to try to make it a bonsai

That makes no difference at all. It stays the same tree species, it will still have the exact same requirements and does NOT belong indoors.

my 9 yr son is thrilled and very enthusiastic

Good. When he is 25, he may have reasonable pre-bonsai. It will take a bit longer to create a real bonsai though.
However, you need to learn that bonsai are not created by keeping seedlings small. You are not growing a bonsai at this moment, you are growing a tree.
Start studying how to take care for your tree first, and how to make it grow - and it needs to grow hard. Spend your time studying the subject. Lear about redwoods, then learn about bonsai. You have plenty of time.

Do not prune, wire, repot it before you know how to prune, wire and report, when to do it, and why.

Not sure what your USDA zone is and if your tree can survive outdoors. You may need a greenhouse...