Young Dancemakers Company brings its 2015 season to a Gala Finale with a 7:30 concert at the Ailey Citigroup Theater.   Like all YDC concerts, the performance is free to the public, but reservations are required.   Call 718.329.7300, ext. 3358 to reserve seats.   Chosen from NYC high schools, the 17 talented teens create and perform their own original works, and also appear in an except from a work by associate director Jessica Gaynor.   Founder/director is Bessie Award winner Alice Teirstein; music director is William Catanzaro.