>>12106736It may be easier, but that's only if you already know that what you're doing will actually work.
Test anti-aging on an unborn, and you're not going to have measurable results for 30 years, and it's really bad if anything goes wrong since you may affect their development and they've got their whole life ahead of them, and of course they didn't consent.
Test anti-aging on an 80 year old, and you've got data pouring in almost immediately, as a lot of the changes would be short-term.
Even if you had to wait an extra couple of decades to modify a whole human instead of an embryo, you'd still get results faster that way. It's definitely not worth trying to make an immortal embryo as an experiment.
As for how you'd modify an entire human, I imagine white blood cells would be easy to modify, and they can access all our cells. Human cells could be modified so that their membrane contains information about what iteration of DNA they're on, and white blood cells will copy their DNA to any cell on a lower iteration or lacking any indication of iteration.
