The only real answer here is that it depends on age (and height, a lot of people seem to be putting hard weight numbers on it instead of relating it to bmi or body fat percentage which is just... sure, retards). Sadly if you want to maintain health you will need to be less fat as you get older.
Someone can handle being close to Tara's age 18 weight when they're just 20 years old (she's even fatter now, of course, she's at a weight where no one is okay). It won't destroy your knees in just a few months to have a BMI of 100, and your organs are still regenerating too quickly to impact you.
By 25 your body will be breaking down from that much weight, unless you do a LOT of muscle-building. If you assume a relatively sedentary life, you will have major issues unless you're down to a BMI of like 80 by then.
The BMI that your body can handle without destroying itself will pretty much go down by 10 every 5 years after that until you go under 40. Anyone can more or less handle a BMI of 39, lots of healthy & active grandmas do that shit, people drastically overestimate how large the cutoff for "morbidly obese" is. For reference, on a 5'2" woman, a BMI of 39.9 is just 218 pounds. If you wanna live to 80, that's pretty much where you have to cap off by age 55. A BMI of 50 at age 50 is pretty manageable, especially if you grew up fat. But any extra strain on your organs takes an increased toll on you as you get older.