Associate a label with the inputs to get arbitrarily large click targets.

I almost never click on a radio/checkbox input because clicking on the label is just so much easier, and I loathe when a fast-and-loose web developer neglects to properly set up labels for their checkboxes. That said, outside of tech circles, few people are aware that you can usually click on the text associated with a checkbox instead of the checkbox itself, so I'm not sure it would help much for a target demographics of "users over 60".

Unless of course you were suggesting to use labels to implement bigger visual checkboxes, which I gather is what the GP is already referring to with "Currently the only reliable way is to hide these inputs and replace them with images."

Do users know that they can click on the labels?

Even if they aren't consciously aware of it as a rule, it almost certainly helps people who put the mouse over what they're reading, and then try to click the checkbox. Higher chance of success.

I notice when I can’t.