Hey - age catches up with us guys. I'm 50, and I'm good for once, maybe twice in one night. Takes longer to get hard, but it does, and while that's "getting up to par" there, there's lots of interesting things to do :) - I've found that focusing on that while I'm warming up helps the warm up and following play :) :) Stay away from smoking. I'm certain you know that alcohol does great for the spirit, not so good for the physical side. Forget herbs: they only guarantee that the thing that gets bigger is the seller's pocket book.
Just remember, not every 20-something wants or needs "multiple rounds". If it's done right, some of us are perfectly fine with one great round.
The only thing I could think of that would make the sex better in that respect for you would have to do with testosterone levels.
My bigger concern is why you're seeing women almost 20-years younger than you. I sure the physical side iss great and all, but it must get lonely when you make a great joke referencing something they don't have a clue about. Sex will only get you so far . . . a relationship has to have more to truly be satisfying.
I dont think there are herbs that will do that. Im sure you will spend a bunch of money finding that out though, sounds like your mind is made up.
Id suggest kegel exercises to keep the blood flow to the area good, a clean diet, and some red wine. Im 40 and I get rock hard with that routine with no real foreplay required, just some making out. I can usually do it 2x a day no problem, but the whole 'multiple rounds' in succession hasnt really been in my book since I was in college.