The masters divisions at IPF raw worlds have been lifting over the past few days. All of the footage is available on the IPF channel on Youtube. You should take a look to see what the best 60+ tested lifters in the world are capable of.
As for your question, I'm pretty sure there are no studies of this in the context of powerlifting, and the answer would depend primarily on how strong the lifter was when they were 25-40. A 60 year old who takes up formal weight training in his 50's could well be stronger than he ever was at 25-40. A competitive lifter at a decently high level at 25-40 is almost certain to total less in his 60's, but the best way to know for sure is to keep training to be as strong as you can and see where the chips fall.