aging query

It's not an unreasonable way to read it...

But for 13 and 22+ the text does say characteristics which suggests spreading them around is intended. If I have 2 (0) decrepitude and get a 13 or 22+ total, I need 15 points in aging to get to 3. If it were the case that sticking them into a single characteristic was appropriate, the text would just say that and remind the reader that aging points above the absolute value are ignored after adjusting the characteristic.

You can acquire a bonus of 8+ using only the rules in the core rulebook.

It is a nice offset to use for Difficult Longevity Ritual; you can still do quite well out of longevity even with that flaw if you leverage other aging bonuses. It just requires more effort, and as such a good chance of more stories.

I think it implies you can split them, but not necessary that you have to split them. Someone who had 0 (3) and only would receive 2 additional decrepitude may decide that he wants to avoid putting them into the same characteristic so that no characteristic is lowered.