Telomerase gene therapy seems to have extended mice lifetimes and improved health and age related diseases. They applied the therapy to adult mice, not embryos, and they used a DNA modified virus to deliver the therapy. The first linked article mentions an average life extension of 24%. The original article (EMBO Molecular Medicine) is here.
First Gene Therapy Successful Against Aging-Associated Decline: Mouse Lifespan Extended Up to 24% With a Single Treatment A new study consisting of inducing cells to express telomerase, the enzyme which -- metaphorically -- slows down the biological clock -- was successful. The research provides a "proof-of-principle" that this "feasible and safe" approach can effectively "improve health span." [article]