Board Thread:Wiki Discussions/@comment-25358381-20140918020735/@comment-4728499-20140918123730

I'd guess about 50-55 as the point when most ninjas start declining in ability, its hard to say since the most powerful ninjas have tended to die before a decline became obvious, but Jiraiya was able to go up against the 6 paths of pain and incapacitate 3 of them, also Orochimaru stated that Hiruzen would've killed him had he been 10 years younger.