Probably best to let sleeping dogs lie - DLA will continue to be paid until the assessment process for PIP is complete, so there's no advantage to filling in the forms early. Use the time to start collecting evidence and read up on how best to complete the forms.
The year is for existing claims up for renewal. It's also possible there might be a backlog, which means that when the child turns 16, the forms may not arrive then.
I wouldnt reference FOS, just give them 14 days to respond then if no refund, issue the Letter Before Action giving a further 14 days, then you issue the claim via mcol