Hi KV,
Anyone claiming DLA who was under the age of 65 in April 2013 will be 'invited' to claim PIP at some stage, even if they are over the age of 65 by then.
People who are on PIP when they reach State Pension age will continue on the same rate of PIP, pending periodic reassessments.
Although there is an 'ongoing' award of PIP, the nearest it gets to 'indefinite', we have seen far more members getting awards of a few years in duration, even those who were on long-term DLA previously.
Nothing on this board constitutes legal advice - always consult a professional about specific problems