Is a slight bow before elders as an expression of respect—which is reciprocated by placing hands on the heads of youngsters as an expression of affection—considered shirk of respect?

It is not permissible to bow or incline when meeting someone, whether a scholar or otherwise, because bowing (Ruku') and prostrating (Sujud) are only allowed for Allah, the Almighty. The Prophet, peace and blessings be upon him, forbade inclining as a greeting. Even standing for other than Allah is prohibited, so how about bowing and prostrating? Ibn Taymiyyah clarified that inclining, lowering the head for dignitaries, kissing the ground, and similar actions are among what the Imams have forbidden. Thus, Ruku' and Sujud are the exclusive right of Allah.