So what you’re asking is why do non-black people automatically jump to race, right? Because I know for a fact I’ve heard black adults do the same about black teenagers (whose race they too make a point of mentioning, in a derogatory manner without seeing how it is, in a way, counter-intuitive). So, why do non-black people automatically jump to race in the example you give? Because race, to them, is more salient of a difference than age and most people aren’t properly taught to check their racism and biases.