Sudden onset glute wasting is a concern. I'd second going to the doc. Having said that, probable culprits are age related muscle loss (though I'd expect that to be more gradual), wasting related to arthritic changes in the hip joints or wasting due to nerve compression in the low back (the compression can come from disc issues, degenerative changes in the spine related to wear and tear or a combination of those two things).
No matter what the cause, working on glute strength is an important part of addressing the issue. Squats and lunges are great but proper form is very important to prevent knee or back irritation. And there are a ton of other glute exercises out there. Working with a PT or personal trainer might be the best answer.
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