You are in the best position to judge your health, fitness and strength.
Trouble is, no one has a crystal ball. While the OP might be fine now, who is to say what's going to happen in 5-10 years? While technically any of us can have something go wrong health wise at any time, the chances increase substantially as you get older.
For someone female currently aged 65, on average you'd expect to live for another 20.9 years, so to 85.9. However, few people get to that age in perfect health before suddenly dropping dead, so the more interesting question is at what point health starts to fail to the point that having a doberman is no longer feasible (and how much longer you'd get with a smaller, less powerful, easier breed).
TBH as a healthy adult several decades younger than the OP, I wouldn't have a doberman, because if something went wrong I wouldn't be able to control it - and I'm thinking in terms of the doberman seeing a cat on the other side of the road that it wants to chase. I'd probably "just" be dragged into traffic, but an older person would be much more likely to sustain substantial injuries from the fall alone. A smaller dog wouldn't have the strength to pull you over unless you were really very frail.