Everything happens in life dependent on Time. Like you cannot have a child after you reach let us say 65 years of age. And a woman cannot conceive after menopause. And similarly marriage has to take place before the urge for it diminishes. So there is nothing wrong to say that one should get married before a given time. And astrology provides some useful combination of planets and the time when it happens when we can look forward to marriage.
There is lot of logic underlying Astrology which you cannot understand without researching on the subject. Your hearsay axioms do not necessarily be valid ones. Please read my articles on http://www.navagrahastro.blogspot.in