Hi Scottiebronte, good question. The Department for Work and Pensions have said that they 'write to everyone who was aged 16 to 64 on 8 April 2013 (the day PIP was introduced) to tell them that their DLA is ending' in their factsheet, which you can find here.
I've done some more digging for you, and one of the DWP policy documents says that 'PIP will not be available to new claimants who are over the upper age limit (age 65 or State Pension age, whichever is higher), except where they had a previous claim to PIP or DLA which ended within the last 12 months or where they applied before that age, but that claim was not decided. Anyone making a new claim over the upper age limit will instead need to make a claim for Attendance Allowance.'
As yours is not a new claim - as you currently receive DLA - I would think that they'll invite you to apply for PIP, even though you're over the usual age limit.
I hope this is helpful, if you'd like to discuss further, you can give our helpline a ring on 0808 800 0303.