The age categories given on Revised Rulebook p. 19 do not match those given on Monstrous Manual p. 353. The Monstrous Manual prescribes flat penalties of −2, −4, and −6 to all attributes for adolescent/middle-aged, juvenile/old, and baby/ancient, respectively. But the Revised Rulebook prescribes different, more complicated attribute penalties for adolescent (called “youth” here), middle-aged, old, and ancient, and has no rules at all for baby and juvenile.
The Revised Rulebook also does not give age thresholds for the juvenile age category, while the various man, dwarf, etc. entries in the Monstrous Manual do give those thresholds.
Are these discrepancies intentional or accidental?
There’s no discrepancy. To quote the text in question:
The age adjustments above were actually developed by applying the normal effects of aging (ACKS II RR, p. 19) to a monster with default ability scores of 9 in each characteristic.
If you are using attributes for the monster (p. 349), then the aging effects for AC, speed, HD, and damage can be calculated by applying the appropriate penalty (-6, -4, or -2) for the age category (Baby/Ancient, Child/Old, and Adolescent/Middle-Aged) respectively.
It directs you to the RR as the basis for this rule, which is “for AC, speed, HD, and damage” - that is, the effects of STR, DEX, and CON, which do neatly follow the -6, -4, -2 breakdown as listed. Rules for babies and juveniles are not included in the RR because the RR deals with the creation of characters for play, and no class has a starting age range that would permit baby or juvenile characters to be created.
the effects of STR, DEX, and CON, which do neatly follow the -6, -4, -2 breakdown as listed
That’s true for middle-aged, old, and ancient, but is not true for adolescent/youth, which gets penalties to STR, INT, and WILL but not to DEX or CON.
Hmm, it does appear so. It is possible that’s a humanoid vs monster distinction, I will reflect on that and see what makes sense of that. I would definitely still follow the monster age adjustment table for non-humanoid monsters, but I could see it going either way with young humanoids.