SNP did the same in Scotland. It's obviously a tactic to try to win more votes as 16 and 17 year old are more likely to vote for more extreme policies than more centrist parties (in the case here independence vs not independence). The irony is that their guidance in the court system is to give more lenient sentences to under 25's because they haven't fully developed mentally.
It's an odd choice from Labour though. I don't see many 16/17 year old voting Conservative, but can't see them going for Labour either. I think less centrist parties like the Greens and Reform will benefit.