Stylistically you can use either and add to the way you speak: for example if you said "he was a beautiful boy." you can imply fragility, adolescence or femininity or if you said "she was a handsome woman", though most would take it as an insult if you said it about, say Frida Kahlo it might imply an appreciation for power or masculine beauty within a women. In short: most English adjectives are gender blind, so technically both ways are correct.

Girl: Pretty, good looking, glamorous, alluring, gorgeous, cute, enticing, stunning, sublime, lovely, comely, dazzling, exquisite, ravishing, angelic, fine Boy: Dapper, fine, stately, suave, pulchritudinous, robust, majestic, comely, clean-cut

For boys, we could say words like "suave" and "dashing". For girls, we could say "pretty", "elegant" and "alluring". There are also words that could be uses to describe both genders, for instance, "good looking", "gorgeous", "charming" and "attractive".