You asked
What is the amount of tax contributed by the over 65s in total and as a proportion of all income tax?
We said
We do not hold the information to answer this query. To be able to answer this question, we would need a breakdown of the value of income tax into the amounts paid by individuals aged over 65. We are currently looking to acquire the data that would let us calculate this value.
HMRC publish some information about (a) the number of tax payers by age bracket and (b) the value of income tax paid by different groups, which is available here. To our knowledge, the breakdown you seek is not publicly available at present, but HMRC may be able to advise further.