What are the best areas to stay in Seoul for nightlife? Hongdae vs Itaewon vs Gangnam

I'm 25 and coming to Seoul mainly for the nightlife scene — clubs, bars, live music. I've heard Hongdae is for younger crowds, Itaewon for internationals, and Gangnam for upscale. Which area would you recommend for someone who wants to experience all three? And is it easy to move between them late at night?