Well for Luffy and Law it’s already been stated that their lifespan is affected by devil fruit powers but then what if devil fruits affect everyone who uses them as well? Well here’s a few points:
1. Whitebeard is one of the few characters we’ve seen in the story who was actually dying of old age, and he was a devil fruit user. He was also not the oldest human in the world still.
2. Kureha is the oldest person in the world and she doesn’t have a devil fruit. She’s said she owes it to good nutrition and the like but still, a naturally good lifespan is also very likely to play a roll.
3. The World Nobles who seemingly have access to some very strong devil fruits aren’t shown to actually possess any powers as of yet. We’ve seen a bunch of Nobles and Royalty and none except Doflamingo has been show to have a devil fruit.
4. Doflamingo quite interested in the Perpetual Youth Surgery. It would make sense that someone who knows the downsides of devil fruits would immediately seek to remedy said downside by seeking extended lifespan.
5. Vegapunk who also has access to many devil fruits and even managed to replicate devil fruit powers like the Pika-Pika fruit still went out of his way to also make Pacifista to be the ones that used the powers. It’s like he knows the use of Devil fruits would drastically decrease a person’s lifespan and so it’s better to give it inanimate beings or objects like Spandam’s Sword that don’t actually have a lifespan.
6. The Vinsmokes have never really ventured into devil fruit use. You’d think they would want to increase their strength as much as possible but none of them or Germa 66 is shown to possess devil fruits and seeing as they have noble backgrounds and links to Vegapunk, they probably know devil fruits are a bad idea.
7. Why does eating two devil fruits kill a person. Is it a like a poison or something? Or is it extreme rapid depletion of Lifespan to the point of instant death?
Well Marshall D. Teach is the exception to everything I just said and I think Oda will tell us why.
And of course Oda has also given the obvious drawback of devil fruits removing one’s ability to swim and I for one say this is genius at work. He has given us something that is obviously the reason as to why devil fruits are undesirable so that there’s no reason to search for another reason while he slowly integrates this other reason as the story progresses.
*Theory by ImmaIvanoM