Father William somersaulting in through the door details

‘You are old,’ said the youth, ‘as I mentioned before,
    And have grown most uncommonly fat;
Yet you turned a back-somersault in at the door—
    Pray, what is the reason of that?’

‘In my youth,’ said the sage, as he shook his grey locks,
    ‘I kept all my limbs very supple
By the use of this ointment—one shilling the box—
    Allow me to sell you a couple?’