The imperative message in the link you supply is this:
Of the 4.1 million claimants, those aged 50 and above accounted for more than half.
Just under three in ten were aged 30 to 49, while 17% of claimants were aged 16 to 29.
Which would suggest that those over 50 claiming the benefit and a few below that age, have been moved from the old system, DLA, to the new system, which is PIP.
And don't get me started on the old 'adjusting for today's inflation rates' nonsense.