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.