Sim, serve para adultos (homens e mulheres) também. Na verdade, qualquer idade. Veja o que dizem nativos do BrE e AmE.
''Informally, in BrE, we often ask "Who's the birthday boy / birthday girl?" This is irrespective of the age of the 'celebrant' (a word never used in that context) - they could be over 80!'' [Wordreference - UK]
''Yes, in the States we would say "birthday boy" or "birthday girl" too. This is more likely to apply to young kids, but with a smile it can be for boys and girls of any age!'' [Wordreference - USA]