I've now linked to 2 studies (and there are more) supporting my statement that "most WR's start to decline by 31," along with taking a list of all-pro WR's someone posted to show strong years at 31 and showing that all those WR's had their best year before then (other than Rice in non-TD terms). That may not be perfect support for the odds favoring WR's under 31, but they're better than these:
Both of those studies have the same problem, which is that they are comparing incomparable WRs. TJ Houshmandzadeh had a decline at age 31, but he was never as good as Roddy White in the first place, and his QB changed from Carson Palmer to first-year starter Ryan Fitzpatrick when TJ was 31. To suggest that Roddy White will decline at 31 because TJ Houshmandzadeh did is absurd, but that's what those studies are doing by not controlling for WR quality and situation. White is a top receiver, in a good offense, with the same high-quality QB as he had last year.So here's an attempt at controlling for quality and situation. This is the list of WRs who scored at least 160 fantasy points (White scored 177.6) at age 30, who had had at least 4 1000-yard seasons to that point (White has had 5 in a row).
Roddy White
Terrell Owens
Tim Brown
Plaxico Burress
Mark Clayton
Torry Holt
Steve Largent
Jerry Rice
Anthony Miller
Jimmy Smith
Rod Smith
Cris Carter
Marvin Harrison
Randy Moss
Wes Welker [*]
[*]Welker is also 31 this year
This is the population you need to compare to. Not Curtis Conway and Troy Brown.
So, how does it look? Of the 14 (excluding Welker),
10 of them scored over 160 points at age 31, and Joe Horn scored 159.5. Three of them scored over 200 points (Rice, TO, R.Smith). The downside exceptions were Plaxico (hold out, suspension, injury), Mark Clayton (injury), and Anthony Miller.
I challenge you to find any football population that does better than this in year N+1. I guarantee you that 23 year old second-year WRs don't repeat or improve on rookie stats at a 78.5% rate. So what the stats are saying, if you look at them correctly, is that a top-quality 30-year-old WR is nearly a guarantee to produce a quality season at 31. [*]
[*]Except for the other fallacy that these studies fail on, which is that observations of populations don't predict individual success or failure.