Trying to remember a book where people get gene therapy or something that somehow tricks their body into believing it's 25 years old. The concept is very similar to the Longevity treatment in Kim Stanley Robinson's Mars Trilogy or in the SubLife in Jessica Chiarella's And Again.

But the difference in the book I'm trying to remember is that in this treatment, they also guard again future illnesses and diseases, effectively making the person live forever, barring some sort of accident. And one of the protections against that was lining thier skull with some sort of something like titanium.

Anyway I hope that's enough to go on. I read a lot of Robert J. Sawyer at that point and I feel like it's one of his books but I can't remember.

Any help is appreciated!