Strand was amazing at the beginning, middle and end. Very few people maintain that high level of excellence, although Strand took about ten years off to work as a film maker, during which time he did very little still photography. Szarkowski said that Ansel Adams had about a 20 year peak before he began to fall into repetition.
Emmet Gowin (whom I met last week ) did great work at a young age and has been consistently very good. He's now 72 or so, still making great photographs and obviously very passionate about it. (His wife is lovely and looks nothing like the classic images would suggest.)
Sugimoto and Izu are doing amazing work in their 50s and 60s. Keith Carter, too. Carter didn't actually become known until about 40. Frederick Sommer was great right until the end.
Don't forget Avedon's "In the American West" which he started at age 56 and completed around age 62. Robert Adams didn't even pick up a camera until his 30s, and he's still going strong past 70.
At 44, I personally refuse to believe that my peak years are behind me. Maybe someone needs to slap me. So much of what you see out there in PDN or American Photo tends to focus on the youth of photography, but most of it is uniformly uninspiring 35mm digital crap. It all looks the same.