I'd imagine the younger knights being the more fit, seeing as they would've plied their trade more vigorously in search of land, titles and wealth; followed by the grizzled veterans who were still active- the 'fat old knight' I figure is the one in semi- or full retirement, enjoying the fruits of his labors (and those of his serfs, if applicable). Still, that fat old knight will not have forgotten how to maintain his seat or how to use his sword...he'd still be a formidable opponent in my book.
Seeing as we live a good deal longer than our earlier counterparts, I'd consider a knight less than 60 years of age to still be hale and hearty, youthful even. William the Marshal still led forces when he passed into his 70s, and that was over 800 years ago; what was exceptional for his time could have become the norm if we still lived medievally with regard to knightly pursuits.
For myself, I'm somewhat slower and less agile than I was in my 20s, but am a good deal stronger and more centered; my workout routines have changed over the years, from being upper body centric to a whole body ideal which is present in my current routines. Seeing as my workouts were haphazard at best in my youth, I can understand why I see the strength and endurance gains I'm seeing now. It used to be wearing a 35 lb harness (mail coif and hauberk) would wear me out in about 90 minutes, but this year, my first outing had me in a 3/4 harness weighing about 40lbs for over 4 hours and no subsequent joint or muscle aches of any kind. Sweated alot, but I felt great!