I've worked on and off on this sort of thing for a while. I'm a busy guy but I'd be happy to give you some pointers if you ever want to contact me.
The solution is rather easy... you have to mod one of the pre-existing cultures. You'll find this is the biggest issue with any mod in CK is the limitations, and it is why main of us wait with baited breath for CK2. Basically you have to change the config files to change an in-game culture to "Elven". Easy, right? There are a couple unused ones... Iberian, Celtic and Byzantine come to mind.
Then, to prevent their death, edit the death by old age event
s (note the plural) in the character_health_events in the db/events folder to not include people of that culture. It should look like this (I think, it has been a while):
Code:
trigger = {
condition = { type = age value = 45 }
condition = { type = not value = { type = culture value = byzantine } }
}
The best source of knowledge is checking out the other events. I haven't written a CK even in years, but I can piece one together using other events in the events folder.
Edit: Suggestion, make the fertility of elves really, really low. Like almost infertile. Maybe a trait will help with this. Or they'll have a million children, half a million will be boys, and that is a lot of prestige loss unless you have enough titles to hand out.