I think the percentage of UK between 16 and 18 that are Muslim is going to be more than double that of the percentage of the population above 18 that is muslim. Complete guess without researching but I'm guessing some in Labour see this as a way to accelerate demographic change in the voting population.
While I wouldn't put anything past anyone, I don't see this moving the needle much if that's the goal.
Some quick Googling tells me that (rounded off numbers) 6% of the UK population is Muslim, 10% of the 16-17 population is Muslim, and 2% of the UK population is 16 or 17.
Making some huge round numbers and assumptions, the electorate goes from 6% Muslim to 6.08% Muslim. I found some surveys that suggested 60 - 80% of Muslims vote Labour, so assuming that 16-17-year-olds would vote the same way and at the same rate (probably optimistic), you're talking about increasing their vote share by around 0.05%.
I would think that they are after 16 and 17-year-olds generally, not merely trying to chase 0.05% of the vote from Muslims.