The only way to change an age or to find out what symbols were added to an age is to do direct NBT editing. There is a DAT file for each age under \worldname\data\agedata_#.dat. Open that and you can see/possibly edit both the pages and symbols used. I haven't tried editing one myself yet so I can't say if it'll work for sure.
PROTIP: MAKE A BACKUP FIRST.
Also of note: The descriptive book contains no actual data about the Age itself, it's only a reference to it.