Unfortunately, in the vast majority of cases age is a great barrier to set in place to ensure a more mature police force. Yes people still slip through the cracks, but it is the best system we have. There is no way to tell maturity from a single interview and we already reserve the right to deny anyone we think would not be a good fit for the force during the interview process. We don't want squeakers slipping through either.
Honestly, the age limit could probably be higher and it would improve things, but 17 is a pretty decent bar as it stands.