Watch out for what people here say about mabinogi, half of them don't know what the **** they're talking about.
But yeah go with what Greed says, it's reccomended and it can be at any time, doesn't have to be 15 weeks.
I agree with you alot on that.
But like it's been said, not required, but it's a good idea. I'd say it's best to wait more then just 3 weeks to rebirth though. Rebirthing every 3 weeks is a good waste of ap that you can get from aging.