You both need to take out POAs. We have had them in place since we were in our 40s. Old age is not a prerequisite for sudden (or gradual) incapacity.
DH and I have had to deal with the last years of four couples in our families and the two who did not have POAs caused us endless problems.
A particular problem you will face (we did) is where all bank accounts are joint and where the couple name different attorneys. Usually couples share trustees, including each other. In your case, as your DH is diagnosed with dementia it would be unwise to name him on your POA. Make sure you both name the same second attorney on both POAs, a child, trusted friend or, if these do not exist, a solicitor. Cheques on joint accounts must be signed by someone representing both account holders.
The alternative is to have different trustees but separate your finances and have separate bank accounts.
Also make sure that your trustees can exercise their power jointly and severally. A friend had a POA for her mother jointly with her husband, but not severally. Her DH died suddenly. This immediately invalidated the POA because one of the signatories had died and it was joint only. On top of everything else associated with her DH's death, our friend had to sort out a new POA for her mother, who by then had severe dementia, so she had to go through the Court of Protection.
But as others say talk to a solicitor. Do not discuss it with the bank, as many can tell you, they are still culpably ignorant about the law and circumstances surrounding POAs.