Many thanks for your patience while I looked into DWP rules. Not easy to find so they clearly don't want us to know.
Hopefully the following will clear up any problems
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If you are awarded either rate of the PIP mobility component before your 65th birthday or receive an award on reassessment from DLA to PIP, you continue to receive it beyond age 65.
If awarded the enhanced mobility component, it is only if your mobility needs reduce that you may move to the standard rate ( and back to the enhanced rate if your mobility needs increase again within a year).
This information is confirmed in section 8.1 of Factsheet 87: Personal Independence Payment and Disability Living Allowance
Although you must make a PIP claim before you reach 65, once you are awarded PIP, it can continue to be paid after 65 as long as you continue to satisfy the entitlement conditions.
If you have a short break in entitlement (less than 12 months) after the age of 65 and then become entitled again as a result of the same medical condition, you can make a new PIP claim
If you are 65 or over
You cannot normally start to receive either rate of the mobility component after the age of 65, although you
continue to receive it if it was awarded before your 65th birthday.
If awarded the standard rate of the mobility component before you reach
65, you cannot move to the enhanced rate if your needs increase after you reach 65
If awarded the enhanced rate of the mobility component before you reach 65 and your mobility needs reduce to a level that qualifies for
the standard rate after 65, you can move to the standard rate (and back to the enhanced rate if your mobility needs increase again within a year).