Traditions run deep in law enforcement. We are neither the most subtle nor the most politically correct for that matter of all the professions. Rookie is an accepted title (like it or not) and it's police tradition. Most believe that it is a derivation from the word recruit. There are several ideas about its origins, but any way you cut it, it means that you are the apprentice. Neither the journeyman nor the veteran, rookie is your title.
When I came along, anyone was a rookie as long as he was a patrol officer. Then when he was promoted to advanced police officer (one stripe!) he lost that moniker. Speaking to other colleagues of mine, some departments left you with the title until another officer was hired to your junior. In some departments, this could be a long time.
I have heard alternative terms such as probationary officer, conditionally appointed trooper (CAT), recruit, kid, and cadet, and there must be another few more out there. And I cannot forget the ever popular FNG (Frickin' New Guy). "Young Pups" is used in some areas, which means rookies are like young puppies. They yip and yap and chase cars. Hey, I don't make this up; I just report what I hear.
It matters not what the title may be; your actions may cause colleagues to call you a rookie. If you mature and conduct yourself as a seasoned officer, this title will eventually disappear from around you.
I have seen some officers with several years under their belt make a boneheaded mistake. The seasoned officers will mumble "rookie" behind their back. In that same department an officer who is junior will handle business and never be called any name. It is all in how you conduct yourself.