I heard that drinking a lot as an adolescent will damage brain development. what about just moderate drinking?

Yes: Research shows that alcohol in any amount is toxic to the developing brain of a child or adolescent.

The definition of : "moderate" alcohol consumption is very variable & its effects depend on genetic influences, gender, age at initiation, & pattern of drinking. A "binge", 4 drinks for a female, 5 for a male, is most harmful, as increased degree &/or frequency of hangovers &/or withdrawal predict worse scores on tests of visuospatial processing, sustained attention, learning & memory, working memory, & planning.