In the running world there is certainly data on age vs. performance. The laymens version is discussed in the book "Born to Run". The cliff notes version is that performances increase from 18 to peak at 27 and then begins a decline. The decline is incredibly shallow however and doesn't reach the same level as an 18 hear old until they are something in the neighborhood of 70 years. I forget the exact year but it's somewhat stunning. So Horners age related physiological limits should theoretically be the same as they were when he was 25 or 26.
The running world is full of incredible performances by older athletes. In 1995 a 55 year old Tarahumara Indian won the Leadville 100. The course record (by 30 minutes) is held by Matt Carpenter and was set 9 years ago when he was 41. Last year at the Run Rabbit Run 100 in Steamboat Springs, Karl Meltzer won at age 44. This was at a race that had the largest cash prize ever offered at an American Ultramarathon by a long shot so the competition was fierce. This very morning Metzler, age 45 took second and beat his time from last year by almost an hour.
In ultra-running racers don't need to be on a team and get a contract to keep racing at the highest level. All they need to do is enter a race so there is much more data on older runners there, many of whom continue running into their 70s. Also they don't generally get paid. Many, most in fact, top races have no cash prizes at all and not runners make a living at it so any doping that occurs has got to be minimal and of negligible effect.
Also the effect of aerodynamics is non-existent in ultra running. Sometimes the difference between 1st and second can be hours. you never see 200 guys running for 6 hours and all arriving at the same time or sprinting it out at the top of a mt for seconds. That also makes for a better data set to look at physiological decline with age.
And of course we can see some examples like Ned Overend and Jeanie Longo in the cycling world who continued performing at a high level into their 50s.
Bottom line, there are many reasons to doubt Horners performance in my opinion but age has got to be pretty low on the list. The advantage he has in freshness certainly must far exceed any miniscule disadvantage he has due to his age.