Depends very much on the person and on previous experience. For us, driving right hand/left hand drive cars on either side of the road, automatic or manual- is just second nature as we have always switched from a very young age (I came to live in London 1 month after I'd passed my test here in Switzerland (CH) - and had to learn to drive in centre of London on 'wrong' side of the road, which was a bit
at first.
If you are not used to driving on 'wrong' side of the road and with a left hand drive vehicle- then I'd say you shouldn't. Coming out of airports can be a nightmare, and so are sea-side and mountain roads. I have UK friends who will not drive on Mway or into town for instance... and really if you are not a confident driver in the first place, it would not be a good idea anyhow, at whatever age.
In the USA, I believe you can't hire a vehicle if you are over 70. Would be interested to hear of exceptions as OH will be 780 next year and we plan to go visit the US again next autumn. Don't fancy do all the driving on my own as we always share normally.