best credit card

"Best" is super subjective. The "best" often carry annual fees of upwards of $500. But another "best" will have no fees, but require a substantial investment account with the issuing bank, or will give fewer rewards.

It's definitely a "what would make it the best in YOUR situation" sort of thing. What are your thoughts on some of the key attributes you want? I would start with no foreign transaction fees and from Visa or Mastercard as my two most preferred "wants" in a travel CC. But I'm also using cards for either airline perks and/or cashback perks.

Ditto what Tom said. If you're not planning to use the card to get future rewards/perks for travel or airline miles, I'd stick with something very simple - Visa/Mastercard (widely accepted in Europe) with no foreign transaction fees. I have a Capital One card that meets those criteria.

With regard to trip insurance, you'll need to look at the details very closely, as usually the insurance offered by credit cards applies only to things you purchase with the card (airline tickets, hotels) and not things like medical emergencies. It's likely a separate umbrella trip insurance policy will be more thorough, but again, you'll have to do some research for your specific situation.

Not to assume, but at 75, you probably are not looking to build a lot of miles or other rewards. The main thing is a Visa or Mastercard with no Foreign Transaction Fees (FTFs). All cards you get now are going to be both Chip enabled and Contactless, both important. If you really want to get a good card for travel, get one that has a PIN for purchases issued with it, though in the US, very few do, though many offer a PIN for cash advances. If you really want to check if a card can have a purchase PIN, check here: https://www.spotterswiki.com/emv/ Here is a search in that database for cards with PIN priority: https://www.spotterswiki.com/emv/cardsearch.php?priority=pin&type%5B%5D=credit

As for annual fees, that comes down to whether the benefits you derive are worth the cost of the fee. For things like Trip Insurance and Car Rental Insurance, the better cards, ones you pay an annual fee for, are more likely to have better coverage. However, at your age, I would highly recommend paying for separate medical and evacuation insurance, and not rely on credit card coverage (Your Medicare may not be valid, at least Part A is not, in Europe.

Maximum age for rental cards varies by country and vendor. Some have limits, some simply require you purchase additional insurance, some have no restrictions. Check with the company you are interested in.

The car rental thing is incorrect. There is no maximum age in France. Take a look at https://www.autoeurope.com/travel-tips/rental-car-age-requirements/. You would have a challenge if you were going to Ireland but no worries in France. I know 80+- year-olds who have rented cars in France.
As to the credit card, I suggest Capital One. My Capital One cards have no foreign transaction fees and no extra charges for using them outside of the US. I have no idea if they offer trip insurance because when I get that, if I am getting it, I get it at the time that I book my trip and I use my American Express card for that.

Capital One also has an online bank account (branded "360") that I use as my back up source of funds while traveling. The debit/ATM card provided with that account charges no fees for use of a foreign ATM, and it rebates usage fees charged by the ATMs themselves. ATMs in Europe (as elsewhere) are increasingly charging fees, sometimes fairly substantial ones. In most countries cash is used much less today than it was 5 or 10 years ago, so most travelers don't need to make many ATM withdrawals. It's still nice, however, to be able to do that without getting socked with extra fees.

My second bank account is a Capital One 360 that I use mainly for travel funds . That account is where I keep my travel funds and my main bank account debit card serves as a backup in case the Capital One debit card does not work which so far has not happened.