I define it as follows: When people live in a vibrant and well-populated tango community and they go out dancing multiple times a week and have gone out dancing for years in their tango community, they become milongueros. They adjust, adapt, and become constituent of the social environment of the milonga.
A milonguero is at the milonga to dance, but only if the music is good and people he or she wants to dance with are there. When not dancing, they socialize. Maybe some people are more socializer than dancer, but a true milonguero is first and foremost a dancer. In one’s home tango community, people know each other’s dancing and they know who they like to dance with. A good and mature dancer knows the music, even knows the music he or she likes to dance to, and with whom. Milongueros do not dance to every orchestra. A seasoned tango dancer does not generally dance to non-tango music. But at a good milonga, DJed by a good DJ, no non-tangos are played (maybe an interlude of salsa is played.) I went out to five different milongas, and like I said, not a single non-tango was played.
In the Bay Area there are probably a hundred milongueros. These are folks that have been dancing for decades. The older folks dance slower or more simply, in general. Talented young dancers who move well enough can break in with the more mature and skilled dancers. Older men and women both enjoy dancing with younger dancers for all sorts of reasons. But true milongueros look for dancers whose movement ability and musical sensibilities match their own.