I touched on this when answering a recent thread, I'll expanded on it here:

In a House of Commons library briefing it was reitterated that they are going to stop 'regular' PIP reassessments for those over pension age, unless they report a change of circumstances:

The DWP also announced on 5 March 2019 that it would no longer be undertaking regular reviews of PIP awards for claimants at or above State Pension age unless they report a change in their needs. The Department expects to implement this policy from late Spring 2019.

Just what this will mean in practice is yet to be seen.

I suspect that all those reaching (or already over) pension age will be given 'ongoing' awards PIP at their next reassessment.
'Ongoing' PIP awards have 10 year reviews.

Why they are doing this is anybodys guess, but it maybe to try and cut down on the reassessment backlogs.

(More cynically it may be an attempt to remove PIP mobility component from all those above pension age, so that it becomes more like Attendance Allowance).