Would Botox in masseter cause accelerated ageing?

Conservative use of Botox for facial slimming in younger patients should not cause accelerated aging.  It depends on how you take care of your skin - if you have very healthy, thicker, elastic skin, you should have no problem with  a few Botox treatments in the masseter muscles.  Actually, there are some published papers that demonstrate some improved skin health and appearance from very tiny drops (microdroplets) of Botox very superficially placed in the skin.  You should discuss your concerns with your dermatologist, but I think there are many more factors that age the skin and cause sagging and wrinkles than some Botox in the masseters.  Best of luck!

If placed tastefully and appropriately then it should not cause accelerated aging. 
It would help ease your TMJ pain and also slim the face if used in moderation 

Good question. Masseter muscle is one of the main muscles of mastication which  is primarily hypertrophied (increased muscle bulk) in Bruxism. Injecting botox in  masseter muscles in patients with severe bruxism, like yours, would not only produce the slimming effect but would also provide  some relief of the discomforts  associated with bruxim like TMJ pain, headache or dental issues. With regards to "accelerated aging", it is a known fact that we loss muscle as we get older which would contribute to less fullness of the area injected. However, when Botox injection is given, it is administered strategically in locations where it prevents muscle contractions thereby minimizing or preventing the appearance of wrinkles. There are muscles that are not suppose to be injected unless medically necessary. This way a patient could appear younger because of less wrinkles. Of course injecting Botox in the masseter muscle repeatedly would  decrease bulk of the muscle, thus producing the slimming effect which is expected. But in patients severe bruxsim the issue is muscle hypertrophy,  which for some, is not only aesthetically undesired but also causing unnecessary discomfort. It is best to talk to a medical professional how to minimize the effect of bruxism, including minimizing hypertrophy of muscles of mastication. This way, you maybe able to avoid using Botox injection long term.Hope this helps.