Everybody has a weakness somewhere. So when do you call it a learning disability?

Impairment. Ld comes out of school observation that some children, despite average or higher IQ have major difficulties with a particular type of task such as reading, even with extra support. The focus usually is on key academic skills--reading, math, writing. I cannot draw nearly as well as many of my peers, but there is no artistic ld. The key impaired learning not explained by iq.
There's a spectrum. Ibest to think of cognition as spectrum: from typical to different to disordered to deficient to disabled. This spectrum explains most learning styles from any perspective. Depending on where one falls on this spectrum, an assessment is made about severity & intensity of problem. Preference should be for one's strengths not weaknesses. We call it a disability when significantly impacts learning.