Do Jehovah's Witnesses maintain 'secret' teachings? I have heard that within the JW faith, there are additional teachings that converts do not learn until after some time has passed since their inclusion into that group or the convert achieves some level of merit within their organization. 

Do the Jehovah's Witnesses keep some of their teachings secret from the general public and from new converts to their faith?
All the teachings of Jehovah's Witnesses are publicly available on their Jehovah’s Witnesses' Official Website and made known in their public ministry and literature.  There are no secrets that take time or merit to unlock.

I have been a Witness my entire life (42+ years), and the things I study and learn now are the same things I studied in my teenage years before I was baptized.

The only thing that might be considered "secret" is the guidebook that elders receive upon their appointment, which is kept confidential and not distributed to other congregation members or the public.  There is nothing in there related to teachings, however, it is merely procedural guidelines for handling congregation matters. Much of what is written there is sourced from other publicly available Witness literature.