Of course 17-year-olds should be treated as children when they are held in police custody. For the perfectly good reason that they are children. As, therefore, are 16-year-olds, who are already so treated.
And children ought not to have the vote. While adult society has a responsibility to protect children from, for example, the predatory sexual attention, which by definition such attention must be, of irresponsible adults.
So, no lowering of the voting age. But a raising of the age of consent.