None of those WRs played well into their mid to late 30s. Cobb is 31 years old right now. Edleman's number's took a nosedive when he hit 33. Welker put up crazy numbers at 32 that Beasley has never came close too, and was not re-signed by NE. If anything, the examples that you provided show that slot receivers pretty much reach their shelf life in their early 30s.