HONG KONG—The rising tide of young political opposition here was on full view in the dense residential neighborhood of North Point, packed with mainland Chinese immigrants loyal to Beijing, where incumbent Frankie Lo found himself in a rare fight to retain his seat on the local district council.

Mr. Lo, a 69-year-old television presenter who won the past two races here in the neighborhood’s Fortress Hill ward uncontested, faced a challenge from a 22-year-old college graduate who was 2 years old when Mr. Lo was first elected....