I have a HDHP with $5000 ded. I am 72 so have SS hence Medicare A. Can I open an individual HSA for my much younger wife who is under my HDHP? $4300 max?

You can’t open the HSA but she can since it would be her HSA. She can actually contribute the family plan limit of $8,550 for 2025. 

What counts is how your wife is covered, not who covers her.  If she is covered by an HDHP that is HSA-qualified, and she has no other coverage, then she can contribute to an HSA in her name.  If the HDHP is family coverage, then her contribution limit is $8300  $8550 for 2025, plus $1000 if she is over age 55. 

I believe the 2025 contribution limit is 8550 

Thank you all for your explanations! I’ll quickly help her open an acct so we can contribute this year. It still seems odd that we could contribute the full 8550 since I’m no longer. eligible to contribute…but I like it!