The age doesn't matter at all. What matters is that they haven't been outside yet and haven't potentially been exposed to the Marek's virus (which could be out in the environment). You can vaccinate them at 1 day or 1 week or 1 month old, doesn't matter as long as they haven't been outside yet. The recommendation is at 1 day old just because that's the highest chance that they haven't been outside yet, so they are still "clean". After that people might start to take them out for short periods, and that may negate the effect of the vaccine. What the vaccine does is prompt the body to build up immunity before ever encountering the virus, so that if it ever does, it has the antibodies to fight. It takes a couple of weeks for immunity to build up, that's why the recommendation is to keep chicks inside for 2 weeks following the vaccine. So that's where the 1 day old recommendation comes from - get them as early as possible while they are still in the brooder anyway, to keep them clean, so that when you eventually want to start taking them outside to meet the world and peck some grass, they'll have protection.
I don't know if I'd feel comfortable with in ovo vaccinations either (I'd be worried about cracking the shell or not doing it correctly or it not reaching the chick), but I've done lots of post-hatch Marek's vaccinations and they aren't hard at all. There are lots of very helpful instructions and videos online. Good luck!