Let me start of by saying Jon Jones started his carreer at light heavyweight, walking around at 219 lbs. He was a realitvely tall, skinny lean lightheavyweight. All his opponents he fought near the start of his career and the begging of his title reign looked larger and thicker than he did.
Jon Jones 23 walk around weight 215
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Ryan Bader 6'2" 27 years old, walk around weight at 230 lbs, started his career at light heavyweight, went on to win both the bellator lightheavyweight and heavyweight titles
Jon Jones 23 walk around weight 215
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Shogun 6'1" 29 years old, walk around weight 228 lbs, started his career at light heavyweight, was the champion before losing to Jon Jones
Jon Jones 24 walk around weight 215
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Rampage Jackson 6'1" 32 years old walked around at 230 lbs, started his career at light heavyweight, former lightheavyweight champion comming off his last reign being 2 years ago
Jon Jones 24 walk around weight 219
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Lyoto Machida 6'0" 33 years old, walked around weight 210 lbs, started his career at heavyweight, former lightheavyweight champion with his last reign being a year ago
Jon Jones 24 walk around weight 219
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Rashad Evans 5'11" 32 years old, walk around weight 230 lbs, started his career at heavyweight, fromer light heavyweight champion with his last reign being 3 years ago
Jon Jones 25 walk around weight 219
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Vitor Belfort 6' 34 years old, walk around weight 225 lbs, started his career at heavyweight, former lightheavyweight champion with his last reign being 5 years ago
Jon Jones 25 walk around weight 225
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Chael Sonnen 6'1" 35 years old, walk around weight 235 lbs, started his career at light heavyweight and drained himself to make middle weight
Jon Jones 26 walk around weight 219
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Alexander Gustafsson 6'5" 26 years old, walk around weight 230lbs, started his career at lightheavyweight
Jon Jones 26 walk around weight 225 lbs
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Glover Teixiera 6'1" 33 years old, walk around weight 225 lbs, started his career at lightheavyweight and was on a 20 fight win streak before losing to Jon Jones
Jon Jones 27 walk around weight 230 lbs
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Daniel Comrier 5'10" 35 years old, walk around weight 250 lbs, started his career at heavyweight, former strikeforce heavyweight champion, undefeated in mma and hadn't lost a round before losing to Jones
Jon Jones 28 walk around weight 230 lbs
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OSP 6'1" 30 years old, walk around weight 240 lbs, started his career at lightheavyweight
Jon Jones 30 walk around weight 230 lbs
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Daniel Cormier 38 years old, walk around weight 250 lbs, started his career at heavyweight former heavyweight and was at the time the reigning lightheavyweight title(lost durring the fight) had 3 defences before losing to Jones
Jon Jones 31 walk around weight 225 lbs
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Alexander Gustafsson 6'5" 31 years old, walk around weight 235 lbs, started his carreer at lightheavyweight later moved up to heavyweight
Jon Jones 31 walk around weight 235 lbs
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Anthony Smith 6'2" 33 years old, walk around weight 225 lbs, started his career at middle weight
Jon Jones 32 walk around weight 240 lbs
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Thiago Santos 6'0" 35 years old, walk around weight 225 lbs, started his career at middle weight
Jon Jones 32 walk around weight 235 lbs
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Dominick Reyes 6'4" 30 years old, walk around weight 225 lbs, started his carreer at lightheavyweight undefeated before losing to Jones
Jon Jones 35 walk around weight 247 lbs
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Cyril Gane 6'4" 33 years old, walk around weight 250 lbs, started his carreer at heavyweight
With the exception of machida and maybe thiago(who by all accounts was a huge middle weight to begin with, it seems all Jon Jones opponenets started their careers at either heavyweight or lightheavyweight, and the ones who foguth at middle weight actually had to drain themselves to make middleweight(see video of Sonnen durring weigh in with bisping) which is something Jon Jones has never had to do to fight at his natural weight class of light heavyweight.
As far as the age thing goes, it also doesn't seem like Jon Jones opponents were very old or past their primes. Most of them were in their early 30s or lates 20s and comming off title reigns, fantastic wins, or even win streaks, Jon Jones was only about 3 years into his mma career at the time too. He started training in 2008 and was champion by 2011 so he was a rookie dismantling veterans in their prime. Also many of the legends Jon Jones defeated fought 10 years after losing to Jon Jones.