New PIP claim

i was 63 in 2013 and been on dla for years due to arthritis in all joints and the killer is arthritis of the lower spine,what i would like to know is it as taken 5 years for them to tell me i have to make a new claim for pip as my dla will end soon,now if i was 64 in 2013 then i would have carried on getting my dla indefiintly i had been on dla indefinitely for years so i can not understand why it as taken them 5 years and how do i stand on this,i have refused a assessment at one of there centres due to not being able to get there because of my back problem ie spasms,flare ups i call them attacks and they lay me flat out in extreme pain for between one hour and up to 3,1/2 days,i can not move and have a carer to help me and all this is on the form i sent to these cretins but still they have said you need to attend the second one or you will lose your benefit,it sounds to me like they have already made there mind up oh and its not as if they can wave a wand and send me back to work,i feel like us senior citizens who are disabled and been on dla for years indefinitely are just easy targets.I also read that pip is for people between 16 and 64 and am 68 because of them taking 5 years were do i stand please regards pete