I love https://www.futurelearn.com/ and
https://www.edx.org/
for quality courses for business and pleasure.
And that is a great point about reducing your risk of dementia.
There is even a great quality MOOC to help you prevent dementia
http://www.utas.edu.au/wicking/preventing-dementia
I highly recommend it as a great source of current evidence based interventions for all ages.