We have been grappling with these ethical issues since the mid-nineteenth century and the beginning of the Eugenics movement. You have obviously done some deep thinking yourself, and perhaps it is time for you to engage with some texts in history and ethics in order to see how to take the questions further. I suggest Diane Paul's "Controlling Human Heredity" and "The Politics of Heredity" (both cheap paperback books) and an essay by Erik Parens "The Goodness of Fragility" widely reprinted in bioethics texts (there are many other bioethics resources, such as bioethics.net and http://bioethics.georgetown.edu/publications/scopenotes/
Children have poor impulse control. They often think the world revolves around them. In general they have difficulty managing their feelings and often end up in meltdown or, worse, in violence. These emotional aspects of childishness are much of what we have in mind when we say that adults are acting childishly. And our main goal is not to insult people but to remind them that in the process of growing up they (should) have learned skills that they should be using now.
You sound as though you are thinking of two choices (1) tell your son to shape up or you'll cut the financial cord OR (2) bite your tongue and keep your promise to help him through grad school. But there are many more choices than this. In fact, there need be no association between your efforts to improve your son's behavior and your financial support of his education. You can make it clear to him that keeping a relationship with you is contingent on mutual respectfulness, and simply walk out of the room when he is disrespectful. If you would not tolerate the behavior from a friend or a partner, then don't tolerate it from him.
Do you think that money is your only "leverage" when trying to influence your son? I hope not: let's hope he has respect for you as a person.
Obviously you are proud of your son's achievements and promise. But I have seen smart, healthy good looking people fail to achieve their potential because of moral/interpersonal flaws such as the ones you mention. You want him to succeed in life, so it makes sense to try to help him change (if he seems willing).
Likewise, I don't think the only options you have is to "observe" or to "manage." Your son is an adult whom you love deeply. There are many possible relationships that are appropriate at this stage of parenting. You can tell your son what you wish for him without trying to directly manage his behavior.
I think you are right to claim that parents have responsibilities towards their children, and do not have the right to raise them "any way they want." Children are not property. The larger moral concern, however, is that the state will decide what children are to learn, and in American society, we are most fearful of that (because of our history with totalitarianism and communism). The law protects the rights of individuals. So parents have a legal right to withdraw their children from state-sponsered education. They also have a legal right to teach them rubbish. However, I would argue that they do not have a moral right to teach them rubbish, particularly if it is rubbish that is harmful (Santa Claus probably does not fall into that category; but Abstinence is harmful rubbish).
The right to pursue certain goods (such as having children, or making money) does not justify using immoral means (such as exploiting women, or stealing) and does not entitle one to success (being a parent, or being rich). There are many ways to try to become a parent (or a wealthy person), some legal and some illegal, some moral and some immoral.
Perhaps you think that the right to have children is more of a right than the right to make money? (Like, for example, basic rights for food, shelter, education or health care.) Even if it was a universal human right to become a parent (which I doubt), it would not follow that there are universal human rights to be a parent by any particular means (such as IVF, surrogacy, adoption etc.)
There are many ways to become a parent, and as those in the adoption community often say, "second choice does not equal second best." I wish you the best.
Your question about "best age to become a parent" seems to be asking about what is in the best interests of the child. A comprehensive ethical assessment of this question may and should include the interests of the parents (as persons, their interests are worthy of moral considerations). As for the best interests of the child, age of the parents is not instrinsically morally relevant. Age may be relevant in some situations if it is a proxy for such things as likelihood of living to raise the child or giving the child a good quality environment (older adults may have greater financial resources, or contrariwise less energy). There are so many things to consider in timing the birth of a child--if, indeed, one has such a choice, which is a recent luxury--that it is often difficult to tell what is in the best interests of a child. It seems ethically reasonable to postpone parenthood in situations such as personal financial crisis, personal illness, temporary physical and political dangers, lack of a co-parent (if one thinks that having at least two parents is in the best interests of the child). Additionally, in this overpopulated world, there is no moral obligation to have children.
Perhaps you are asking the more general question, whether it is OK to live a selfish life e.g. spending one's money on luxuries for oneself rather than giving to others and/or making the world a better place? That's a great question. (In my opinion, some parent in a selfish way, viewing their children as extensions of themselves. There are many ways to be a parent.)
A personal addendum: I became a parent at age 41 (both parents and child, now age 8, are thriving!). "Being selfish" would be an inaccurate and overly negative way of describing my early adulthood, in which I was preocuppied with "finding myself" and did not have the emotional space to be a good parent.