Am certain that a couple of years back I saw something in either CPAG or the Disability Rights Handbook to the effect that a government minister, in answer to a parliamentary question, had given a statement saying that periodic review forms would not be sent to claimants over the age of 65 who in receipt of an indefinite award of DLA. But cannot now find any reference to this.

Did I imagine it?

You didn't imagine it and will find the reference in older copies of the Disability Rights Handbook. Unfortunately the periodic review system has now been replaced by the Right Payment programme which has no exemptions.