My mom is an old lady, does it daily, and picked up the habit in her 60s. She is very much not a superhuman. Her swim group is all the other old ladies in their 60s in the neighborhood. The "gradual" introduction is to start swimming a few short laps daily in the summer, and don't stop when the weather cools. They use a pump to keep the sea from icing over near the peer; the ice can be about a meter thick nearby.
The freezing water daily is really not recommended if you did not started years ago. It is quite popular around here, I do it too and I have already seen ambulances going. It is easy to overdo it. It is all fun and endorphins until it is not.
The age plays a role in terms of more likely heart problems. Not sure how about the adaptation, how it is affected by age. Muscles matter.
Young males end in ambulances too. The emphasis on old ladies here us because frankly old ladies are supposed to be least capable species. I know they can be tough.
But daily freezing water swim without prior training is bad idea. The processes in your body are supposed to run changed for multiple days and if you do it daily it is not getting recovery. Just like with weight lifting every day, beginners risk injury for doing so.
Also, there is no need to put gradual in scare quotes. The gradual as in continuing from summer is the recommended way on top of those daily showers and listening to your body.
