How did you interpret “immature” as “not smart enough”? A baby is not mature, which doesn’t mean he won’t grow up. “Immaturity” means that there’s room for improvement and that that improvement is attainable.
What I mean to say is that this issue stems from your misunderstanding of the meaning of the word “mature” and nothing else.
Assuming OP was an undergrad, it’s totally fair to assume that the prof in question was trying to euphemistically say that they weren’t good enough for the class, especially since “mathematical maturity” is something of a term of art in the field. I don’t think it’s worth putting down teenagers for not understanding the nuances of every single bit of terminology in a field they’re dipping their toes into, the expectation should be that professors be more empathetic to that.
"mathematical maturity" would be a combination of "not smart enough" and "haven't studied enough". Highly talented people can reach high levels of maturity very quickly, people who are just a bit above average can reach a reasonable maturity through hard study, and below average talent means it is very unlikely that the person will be able to comprehend university level theoretical math.
A teacher who doesn't know you will not know if you're not smart enough or just didnt study hard enough, and probably doesn't care much, especially for an undergrad. For a phd student, they may care if they think they've identified a "diamond in the rough" that can be turned into a gem with some work, but that is quite a bit beyond basic linear algebra.
> “mathematical maturity” is something of a term of art in the field
Fascinating, it sounds like "MM" is tacit knowledge, which begs the question why it isn't explicitly taught.
In mathematics, mathematical maturity is an informal term often used to refer to the quality of having a general understanding and mastery of the way mathematicians operate and communicate. It pertains to a mixture of mathematical experience and insight that cannot be directly taught. Instead, it comes from repeated exposure to mathematical concepts. It is a gauge of mathematics students' erudition in mathematical structures and methods, and can overlap with other related concepts such as mathematical intuition and mathematical competence. The topic is occasionally also addressed in literature in its own right.
It's been a long time, but IIRC I recognized that my understanding of the term was vague but that all interpretations boiled down to, if I wasn't ready now I never would be.
I'm not claiming that my response was entirely rational or measured. I'm not blaming the professor, and I'm not justifying my own reaction. I was just saying what my reaction was, in case it shows a pattern of student experiences that's worth addressing.
If you have reasonable people skills, you may well have correctly inferred what the lecturer was communicating: overly questioning the words can be damaging. As I have matured, I have learnt better to trust my intuitive reading of what someone says, especially when I detect that the meaning of their words is divergent from what I believe they think beneath the surface.
Stubbornness, tenacity and self-belief are useful skills for success - perhaps you gave up too quickly? Unfortunately, soft skills are very rarely taught well. We all rely upon our subconscious learning and a good amount of luck to gain necessary skills, and those skills are often learnt by osmosis while very young. I also think the meta-skill of being able to teach yourself soft skills is not common either and it - although we can make an effort to improve that meta-skill even though it is somewhat self-referential.
What I mean to say is that this issue stems from your misunderstanding of the meaning of the word “mature” and nothing else.