A "viejo milonguero" is first of all a contemporary witness of the tango craze in the 40s in BsAs. In other words, he was a bad dancer. From a certain point in time on, however, the younger brothers of the first mentioned group also referred to themselves as "viejo milongueros". They were generic "rock 'n' rollers", but they took the chance to promote tango. This group actually could dance!
Janis also refers to her peers and to those native milonga visitors in BsAs as "milongueros" or "milongueras" (this time without a qualifying adjective). In this sense, I would also be a milonguero, not a "viejo milonguero", simply an old "milonguero". I actually use "milonguero" according to the terms "salsero", "bachatero", "forrozeiro", "lindyhopper" or "balfolkie", although the correct equivalent would be "tanguero".