That's not true. In fact, only very long distances - half/marathons, triathlons, or cycling benefit older athletes. VO2MAX decreases after early 20s. That's why most elite runners PR in the 5k when they are 18-23 years old, and I assume the decline in VO2MAX also effects swimming.
Here a list of the fastest 5k times and the age in brackets at which they ran it:
1. Kenenisa Bekele (22) ETH 12:37.35
2. Haile Gebrselassie (25.2) ETH 12:39.36
3. Daniel Komen (21.3) KEN 12:39.74
4. Selemon BAREGA (18.5) 12:43.02
5. Hagos GEBRHIWET (24.3) 12:45.82
4. Eliud Kipchoge (19.7) KEN 12:46.53
5. Yomif KEJELCHA (21.0) ETH 12:46.79
6. Dejen GEBREMESKEL (22.7) ETH 12:46.81
7. Sileshi Sihine (21.4) ETH 12:47.04
8. Isiah Kiplangat KOECH (18.5) KEN 12:48.64
9. Issac Songok (22.3) KEN 12:48.66
10. Yenew ALAMIREW (22.1) ETH 12:48.77
11. Saif Saaeed SHAHEEN (20.7) KEN 12:48.81
Lagat, ran 12:53 at 26 years old, which is already quite impressive. But he always did low mileage and focused on the 1500, so it's likely he just wasn't at his peak when he was younger, and could have run even faster.
Some, like El G peaked even later, at 28 years, but they also never focused on the 5k when they were young. He could have run faster than his 12:50 had he focused on the 5k many years earlier.