Celebrating one’s 18th birthday is not typically a poignant occasion, but for outgoing Hongkonger Chen Tsz-kin, adulthood began last week with a mixture of excitement and anxiety. At 18 months old, Chen was diagnosed with Duchenne muscular dystrophy, a genetic condition that causes muscles to weaken over time. Doctors told his mother, Michelle Li Yuan-feng, that people with the incurable illness had an average life expectancy of 18.