I took this one specifically https://mensa.dk/iqtest/ and it gave me a lower than expected score. Should I attach any actual meaning to it? I want to become a physicist or mathematician (neuroscience is interesting too) but now fear I'm too retarded to achieve any meaningful accomplishments in these fields.

What's bothering me is that it says the test is accurate for 18-30 year olds, but the brain only finishes its development at 25 (so naturally the same person will score lower when 18 than when 25). Also, 40 minutes and 30 questions seem a bit too scarce to precisely determine IQ, I'd say, as I've heard the tests administered by doctors are longer/more extensive.