ESA/PIP Severe Disability Premium Backpay?

Hello I was recently sent a letter from the DWP asking to fill out a form to see if I was eligible for claiming for SDP. I have been on income-related ESA and PIP since 2014. I have been living with my parents so thought I wasn't eligible, but upon checking I've now noticed that if the person you're living with has a qualifying benefit which my parent does(Attendance Allowance) then you should be allowed SDP. My parent has been receiving AA since 2015 and my benefits haven't changed since then does this mean I should be backdated pay till when my parent started receiving the qualifying benefit? I phoned the DWP yesterday and we filled out the SDP form over the phone but they didn't mention whether the person I was living with had a qualifying benefit they just stated his name will they know he gets AA? Any help is much appreciated

Hello Gordon, the person I live with is my grandfather he started claiming AA since 2015. Does the living with part class as partner or child living with parents? I am above 25 sorry if sounds like a stupid question. Also when I rung the ESA line they said they can pull information from a database to see which benefits people have active

My grandfather gets an underlying entitlement to Carers Allowance for me, I'm guessing its not classed as the full title of Carers Allowance so I maybe they won't count it against me?