Just two years younger than you, Richard, I feel the same - Eh What?. In particular:-
1. If the UK govt. treats EU citizens in the UK badly (I think T. May suggested something on these lines -that EU immigrants' rights of residence won't be guaranteed as they're part of Brexit negotiations), will the French govt. have to see us in the same light? i.e. as "bargaining chips"?
2. When the UK leaves the EU, what will happen to our state pensions? The Winter Fuel Payment, which I thought was protected by EU rules, has gone (but last I heard a challenge was underway). Without EU rules to protect us, will the UK govt. be able to freeze our pension as it does in other countries like Australia?
3. Health cover. Would the UK still cover the basic? If not, where will we be?
As far as I can tell no one can answer any of these questions one way or the other, so I think I am right to worry. I applied for nationality last year, but even if they accept me that still wouldn't protect me from 2. & 3.