Actually, the parents decide when "ready" is. Usually it's when the guy has a stable job, which ends up being in his late 20s.
As for what happens if the son doesn't like the girl:
The thing to remember is that if the son is a "catch" (defined as physically attractive, from a wealthy family, working a high-status job, and other things), he'll have girls lined up and down the block for him. He'll be able to pick what he wants.
It seems like you think that the system pits the son against his family. It doesn't. The arranged marriage is a way for the family to help the son find the highest quality woman possible.
And that's why people choose to participate in the system. Look at it from my perspective. I'm a quiet, nerdy, foreveralone guy. American girls either hate me, or think I'm invisible. Anyways, I can do one of two things: 1) I can get into PUA or whatever it is called, study seduction throughout medical school, and eventually find a 6/10 to marry. Or, 2) I can study even harder throughout medical school, get into a better specialty, and get a 9+/10 from India, because I'm a tall, physically attractive physician from a good family. The arranged marriage gives me a much better chance of securing a high quality wife.