Short answer: there isn't an upper limit. Master's-class ultrarunners finish 100-milers in their 70s. The constraint isn't age, it's training continuity. The data, by decade.
The short answer
There is no documented upper age limit for finishing ultramarathons. Active master's-class runners finish 100-mile races into their 70s and 80s. Gunhild Swanson finished Western States at 70. Cliff Young won the Sydney-to-Melbourne 875 km at 61. The sport has more 50+ finishers each year. The age question is mostly the wrong question, the real questions are training continuity, recovery capacity, and joint health.
By decade
Aerobic capacity declines slowly with age in trained athletes. The bigger changes are in recovery time, strength, and joint resilience.
Most ultra performance peaks in the mid-30s to early 40s. Recovery between sessions is at its fastest. Combined with accumulated training experience, this decade often produces personal bests at every distance.
Performance vs 25-year-old self: +5% to +10% (peak)
Master's class begins at 40. Many ultras have produced winning times from runners in their 40s, the mountain ultras especially favor accumulated experience. Recovery starts lengthening (48 → 60 hours between hard sessions). Strength training becomes meaningful.
Typical performance vs prior peak: −2% to −5%
50-somethings are the largest demographic at most American ultras. Training adjustments: 60–95 mpw becomes 50–80 mpw; recovery extends to 72+ hours between hard sessions; strength training twice weekly; emphasis on Zone 2 base over high-intensity work.
Typical performance vs prior peak: −5% to −10%
60-year-olds regularly finish 100-milers, Western States, Leadville, Hardrock all have 60+ finishers each year. Sub-24 finishes become rare; 28–32 hour finishes become typical. Training cycle stretches to 24–28 weeks. Recovery dictates the pace of progression.
Typical performance vs prior peak: −10% to −20%
70+ ultrarunners exist at most major events. Gunhild Swanson finished Western States at 70. Reidar Heggem at 75. The cardiovascular system can support endurance running into the 80s for runners with consistent training. The constraint is more often joint health and life logistics than aerobic capacity.
Typical performance vs prior peak: −20% to −35%
Notable older finishers
Specific examples documented at major ultras. Master's-class records continue to improve as the sport matures.
Training adjustments
The adjustments aren't dramatic. Most are about recovery, not about volume cuts.
72 hours instead of 48. Two quality sessions per week becomes one. The volume can stay similar; the intensity distribution shifts toward easy.
20–30 minute sessions of compound lifts (squat, deadlift, lunge variants, single-leg work). Counters age-related muscle loss. Should be considered non-optional past 45.
The 80/20 rule shifts toward 85/15 or 90/10. Aerobic-base work continues to deliver returns at every age; high-intensity intervals require more recovery and produce smaller relative gains.
16-week first-ultra cycles become 24-week. The progression is gentler. The body needs more time between volume jumps.
Sleep recovery efficiency declines with age, 7 hours at 30 may equal 8 hours at 55. Protein intake should rise to 1.5–2g per kg body weight to counter sarcopenia.