The voting options are a bit strange. On the one hand there are fixed ages (is there a reason 11, 16, and 18 were picked in particular?) while the other option is the vague statement "the gangs age" (which is what I picked). However, the members of the gangs are of varying age (Spike at least was born at a time when all the other members of the gangs were at speaking age). We don't know with any certainty if the LBT characters level is the same as that of a human, but if that was so I would put them no older than 12 (at most) and more likely than not younger than 12 years. In any case they are kids rather than teenagers and far from the options 16, 18, 20, 30, 40 or 50 (their attitude as well as that of the grownups towards them shows this very distinctly, and so do their physical proportions (ration of the head towards the rest of the body for example).