How old do I have to be for a doctor to agree to do an arm lift?
Your age will probably not be the the most important factor, since you are over 18. Arm tuck/ lift is also known as brachioplasty. Reduction of arm fat (without skin tightening) would be done by liposuction instead.
Personal evaluation by your board-certified plastic surgeon will be needed to answer the question if this procedure is right for you. The factors he or she will consider include your general health and suitability for surgery and anesthesia, the amount of arm fat, the amount and looseness of arm skin and any planned significant weight loss. These factors can affect type of procedure(s) and timing. Your surgeon will recommend the procedures most likely to give you the best result at your current age, or in the future. Best wishes.
When evaluating your arms we tend to look at the quality of the skin (do you have a lot of stretch marks) and the degree of loose skin you have. We also look at how much fat you have in this area. In general, if your skin quality is good and you have some fat, then you would be a candidate for liposuction. If you have a significant amount of loose skin or poor quality skin then you may require excision of your skin with or without liposuction. Obviously you are trading a scar for shape. At times, the scar can be visible in daily clothing.
Your age is not important. What is important is that you have realistic expectations and you are embracing necessary lifestyle changes such as diet and exercise. You should discuss your concerns with a board certified plastic surgeon in your area.