I started mountain bike riding in my 50s after 30 years of road. I found it to be great fun and required a far amount of new skills. I liken road bike riding to cross country skiing where mountain bikes are like downhill skiing requiring your full attention, moving your weight and constantly thinking ahead.
If you ride within your comfort level and keep to ;beginner trails (blue) which are not technical, you should be fine. If you find a part of the trail which might make you uncomfortable, it is no crime to get off the bike and walk it over that section and then get back on again. I have done that many times and even more now that I am approaching 70.
I wouldn’t worry about breaking bones unless you have balance issues already.
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Road 🚴🏾♂️ & Mountain 🚵🏾♂️