Question

In Pokemon yellow version what is the earliest level one can catch a paras?

Answer 1

The lowest level you can catch Paras at in Yellow is the first floor of Mt Moon. It's lowest possible level is 9.

Source

Comments

I just wanted to add to spook's answer that paras' are an uncommon find, they probably wont appear unless you try to encounter them.  In my Pokemon yellow I had to try to get one. hope this helps :).