They take even longer to any electoral payoff (even if it's positive), both because of the legal minimum age and because younger people tend to participate less in elections.
They don't have to physically vote. They just give their ballot slip to a "Community Leader" and he fills them all in for the candidate. See Tower Hamlets as an example.
The only irregularities I know about in Tower Hamlets were (1) illegal at the time, (2) caught, (3) not of a large enough scale to change the result, and (4) were cited as a reason for the change of law forcing all voters to present photo ID at the booth[0].
Also: What ballot slips? You don't get a ballot slip if you're not on the electoral register, and that only happens at voting age. Waiting for migrant's unborn kids to reach that age will always take longer than waiting for their parents, who, again, don't start off able to vote as soon as they arrive.
[0] this in turn was condemned as a way to suppress votes from various communities (because e.g. some of the ID was allowed if you were a pensioner, but the identical equivalent ID for students wasn't).