When is an argument from tradition valid because it is a victimless tradition?

If we are focusing on just the fallacious argument, Person 1 is clearly moving the goalposts . They started with an argument from age then when called out on that, changed the argument.

Besides fallacies, I would argue that Astrology is not at all "completely harmless" and every person who believes in it is a victim of bad ideas.

Person 1's logic is something like, "if it has been around for a very long time, then there is some truth in it."

Person 2 then does a valid reductio ad absurdum to expose a flaw in the line of reasoning ("is there truth in forced marriages, because they've been around for a long time?")

Person 1 then employs special pleading (as Dr Bo points out, this can also be considered moving the goalposts) to avoid conceding the point.

I find myself conflicted. On one hand I guess its ok if it s a harmless tradition. But doesn't false or ignorant belief cross into harmful tradition. As it denies objective reality and is only believed because it makes you feel good. 

You're right - and these are two different conversations. The first is "do traditions have truth in them?" and the second is, "are they harmful?"

A belief can be harmful and false, or harmless and false, or something else. But the effect of the belief is not the same as its truth value.
 

The only truth about Person 1's claim is that astrology has indeed been around for a long time. In ancient times kings (regicide), children, virgins, animals, volunteers, enemy combatants were often brutally sacrificed at astrologically propitious times. Entire civilizations waged wars and other atrocities based on what their priests, sages, and viziers foresaw in the stars, planets, and constellations. There is even compelling evidence that the Gospel accounts of the Nativity and brutal sacrifice (Passion) of Christ was borrowed from ancient astrological metaphor and taught sub rosa through various pagan, Jewish, and early Christian mystery schools. So, it is a false claim (premise) that astrological tradition is harmless or victimless. In fact, it is a tradition drenched in bloodshed and violence.

Ironically, today, the FCC mandates anyone advertising or promoting astrological forecasts be accompanied by a disclaimer that it is for 'entertainment purposes only.' 

Person 2's claim about forced marriages is a weak analogy. In fact, it is a false equivalence, if not a complete strawman distraction. I assume the person actually means "arranged marriage" which is a truly ancient tradition that works extremely well in some countries and cultures. I don't know of any evidence that suggests it is any more or less harmful than our own western conventions of marriage (unless, of course, we include the perverse practice of child brides promoted by certain renegade factions of the LDS). Since early traditions of the marital arrangement were mostly for titled monarchs and upper classes it held a great many benefits for keeping the peace and fostering prosperity.

Forced marriage on the other hand would more likely describe a situation similar to a shotgun wedding which is less of a tradition or belief and more of a familial or social remedy.

Shariah law allows for forced marriages in lieu of rape in times of jihad, but it can hardly be called a tradition in the classic sense, and it certainly is not harmless.