Depends who you ask...
First Wavers, those old Lastnamers who joined when SL was still new, who remember when you had to hand knap your own prims from chunks of digital flint while sitting around the campfire in the hill fort listening to Chief Phil give the weekly pep talk... They assume anyone under 10 yrs in SL is a noob...
Second Wavers, who joined up when SL became fashionable, say 2006 to 2009, having been picked on by the First-Wavers, call those below them noobs. Many Second-Wavers assume their IQ is equal to their SL age measured in weeks, and thus consider anyone under 2 yrs old to be not only a noob but intellectually subnormal as well.
Third Wavers, being the lowest of the Lastnamers, hate and revile Residents, in accordance with the principle of always kicking downwards...
Residents, well they tend to determine noobness by your avatar, your actions, and /or your profile.
Be aware that there are such things as 'Perma-Noobs', you sometimes see these fossils staggering about, looking like they are a day old despite their 10 - 13 yrs in SL
As for stuff like "leaving your lookat targets on" etc, ignore that crud, not all viewers ALLOW people to turn those off without tinkering with debug settings.