I am new to Philosophy SE so I am not certain of the protocol, if any, to answer questions. Feel free to adjust, or tell me to adjust my format, sources, terminology, etc. if it fails to meet the expected standard.
- How come Plato raised this question at all:
How is it that we have certain ideas which are not conclusively derivable from our environments?
- What kind of idea/knowledge I didn't get from the environment and had on my mind from birth?
Plato's Meno and Phaedo explore epistemology, i.e. what do we mean when we say that we know something. An example would be geometry and the knowledge of Pythagorean scholars. Plato, in effect, is questioning his society's knowledge of mathematics.
If we can agree that perfect circles do not exist in the real (natural) world, then it is obvious that the quote by OP from Wikipedia, "How is it that we have certain ideas which are not conclusively derivable from our environments" applies to adults as well as babies because, even as adults, we say things like "imagine a perfectly straight line ...". Yet, straight lines are not "conclusively derivable from our environments".
In other words, Plato knew that mathematical models are not real, and do not exist in the natural world. Hence, his enquiry into our a priori knowledge of mathematics.
So, the reason Plato raised this question at all, is simply because he wanted to know how we know what we know of mathematics (geometry) and other knowledge. This should also answer OP's secondary question, on innatism, what a person (new-born or adult) might mentally possess from birth.
Perhaps a better explanation is this quote, from "mathematics, history of the philosophy of" in Oxford Companion to Philosophy:
Many areas of philosophy owe their beginning to Plato, and the philosophy of mathematics is a star example. It was he who first reflected on the fact that geometers speak of perfect squares, perfect circles, and so on, though no examples are to be found in this world. He thought that the same applied to arithmetic too, on the ground that in arithmetic we study numbers that are composed of units perfectly equal to one another in every way, whereas again there are no such units to be found in this world. ... since the objects were not of this world, our knowledge of them must also be independent of our experience of this world, i.e. it must be a priori.
(Meno dealt mainly with "virtue", whereas Phaedo was his last dialogue with Socrates before the latter was put to death.)