-1, for a couple reasons.
Reason number one would be the usage of exploits by people who have alts. As @ChrisRid said, If I use "Robot_WohMi" who has never collected from a printer or leveled up to build my base, that quite literally gives me 10-15 levels in which the only XP source would be playtime XP in which my base is completely unraidable. Sounds very easily abusable and finicky.
Reason number two.... this won't help new players nearly as much as you think it would. Giving new players a "grace period" won't help them not get raided, it will just mean that instead of raiding a level 1-2 who has no idea how to build a good base, you're raiding a level 11-16 who has no idea how to build a good base.
The best way to help out new players is to literally help them out. Find a player(s) whose base looks like they don't know how to spell "Keypad," and offer to give them pointers on how to make a better base.
@1998Corolla makes excellent points throughout their whole post, but this especially sticks out. I'd put money down that >90% of new players wouldn't check the rules thoroughly enough to find that they have a grace period, and that if implemented, this rule wouldn't serve the function it was intended for.