I'd like to qualify my earlier post lest I be accused of Plant bashing.
First of all Im a big fan of both Led Zep and Robert Plant. I bought all of the LZ albums on release and own most of Plants solo work. Hes a great writer, good vocalist, and always selects superb musicians to record and tour.
Having seen Zep many times between 1969 and 1973 there was a marked deterioration in his voice some time around 1971. Live performances of How Many More Times and Since Ive Been Loving You were markedly different in 1972 and later than they were in 1969-70-71. The vocals were much weaker; his range had diminished.
Im constantly amazed at how many of rocks older figures such as Ian Gillan, Roger Daltrey, and Sammy Hagar to name a few, have managed to avoid vocal damage and sound so strong after 35+ years of singing. Good genes? Luck of the draw? Who knows? That being said, I enjoy Robert Plants work; he is one of rocks most important artists. However, his voice has changed more than many of his contemporaries.