Watching Ep 9 of The Legend of Korra,
Korra witnesses in flashback Aang's battle at age 40 against a powerful Bloodbender crimelord. Aang is not immune to the crimelord's power, but he is better able to resist it than are the others present (including Toph and Sokka) and he is eventually able to close with and Spiritbend the crimelord's powers away from him.
At first I assumed that Aang's relative resistance came from his Avatar spirit, since before winning he briefly accesses the Avatar mode. But then I realized another reason why Aang might be more resistant than most to Bloodbending.
His wife, Katara is one of the few known Bloodbenders in that universe. Though Katara fears and detests the corruption of this power (the ability to control someone else's body as a meat puppet through bodily fluids), it is quite possible -- especially since she would have known for months or years that Aang might have to face the crimelord in direct combat -- that Aang practiced resisting Katara's Bloodbending, so as to learn defensive techniques. Aang after all can bend all the elements, and Bloodbending is basically a variant of Waterbending. Even if Aang could not offensively Bloodbend, he might be able to use Waterbending to protect his own body against the Bloodbending influence.
Why was Aang overcome at first, then? For one thing, the crimelord was using an exceptionally nasty, body-twisting attack obviously meant to subdue through sheer agony. Katara probably would not have been willing to go all-out against Aang, since she might miscalculate (she doesn't routinely practice Bloodbending, since she hates the technique) and seriously injure her husband. The crimelord felt no such love for his nemesis. Aang may have been surprised at the sheer force of the crimelord's Blooodbending.
On a lighter note, it was fun and gratifying to see that the around 40-year-old Toph still enjoyed needling "Twinkletoes" and hear the 44-year-old Sokka happily telling the war story of how he beat "Combustion Man" with his "trusty boomerang." People grow up, but if they are lucky, their core personalities never change.