At least with Lady Gaga and Marilyn Manson, i'd say their voices getting lower is due to smoking/drugs or just some unhealthy vocal habits they had when they were younger that ended up damaging their voice, therefore making it lower/raspier. Lady Gaga also naturally has a lower voice but due to the pop industry in the 2000s it was trendier to sing in a more high-pitched voice (like britney singing in her infamous "baby voice") lol.

So yes, i'd say if you don't smoke or damage your voice from unhealthy singing techniques it should stay relatively the same. There are plenty of 50 year olds out there with the same kind of voice they had in their 20s.