I am answering for my husband here. He would say he doesn't feel like he is 20 again, but the sex is better now at 39, than it was when he was 20. We are having way more sex now, than we did 18 years ago. We are both more experienced, more open to new ideas and we don't care so much about what society may or may not approve of anymore.
When he was 20, it was all about knockin' booties, so to speak, now it is more about spending hours upon hours (nearly every time) exploring each other, and working toward more of a whole body experience, rather than just a genital one.
I do not notice any decline in his performance as he has aged. If anything he wants it more often at 39.