What are some basic stretches I can do for five minutes every morning to keep me flexible as I get older?
The very least everyone should do in the morning is a proper squat, not weighted or anything, just sit comfortable in the deepest and safest squat you can for 15-20 seconds. A few reps will warm up the whole system.
The hamstrings are also generally neglected so the stereotypical yoga move is perfect for that.
Here's a good video from a channel you might wanna sub to:
https://www.youtube.com/watch?v=pqM3hCLIpIQ
Also checkout the guys at CalisthenicMovement, they provide some extra depth if you want to push yourself at becoming as flexible as you can:
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