That is the widely accepted wisdom.
However, if say you have one account that you’ve had for 10 years, and a credit card that you’ve had for a few month, your average account age is somewhere in the 5 year range, so still quite decent.
As always a balance has to be struck: If you are constantly switching all your banks and credit cards, then I think a potential lender will have their doubts when looking at your file, as you look very unstable. But if you are switching one of them every now and again, while everything else is stable, then I think that looks like you know what you are doing…
As for section 75: not a lawyer, but as far as I understand it shouldn’t matter. You bought the thing on credit, and that makes the lender liable: Whether you have since cancelled the agreement and/or paid off your debt doesn’t make that liability go away.
Here is what MSE has to say:
If you’ve paid for something on a card but since cancelled the card you may still have Section 75 protection.
The Financial Ombudsman Service told us that assuming you meet all other criteria you should be covered, as the fact you’ve cancelled the card doesn’t matter. The original payment method is what’s key, ie, you paid on a credit card.
If you have any doubt, put in a claim as the worst that can happen is your claim is rejected.