This is the first year that 16- and 17-year-olds can join the city’s community boards, the local advisory groups where the civic-minded, the concerned and the community’s grumblers do battle against encroaching developers, vet liquor license applicants and air block-by-block grievances. Nineteen of them were appointed as unpaid members after the State Legislature lowered the age minimum from 18 last year, an unusual privilege even in a country taking small steps toward expanding youthful civic engagement.