Speed, reflexes, the ability to take a shot are all things that go downhill once you've been in the game long enough
Izzy is a counter fighter. He plays that game of inches, where you move just far enough away to avoid the strike, while still being in range to throw back.
His style is going to suffer most from aging, even if his technical skill is still there.
There's things he could get away with when he was younger that he won't be able to risk anymore, but those are already deeply ingrained patterns that worked because they played off his natural reflexes.
They're not something you can really change easily.
When you watch some of Izzy's fights during his comeup, he was a monster. The speed, reflexes, counters, technique, diversity of strikes. Can't do that forever tho, it's a young man's game.
Hes been fighting for 15 years, not just in mma but also kickboxing. It all adds up.