I'm puzzled that you describe this as a hypothetical.

For example, the culture I live in is pretty confident that five-year-olds are so much less capable than adults of acting in their own best interests that the expected value to the five-year-olds of having their adult guardians make important decisions on their behalf (and impose those decisions against their will) is extremely positive.

Consequently we are willing to justify subjecting five-year-olds to profound inequalities.

This affects my ideas of equality quite a bit, and always has. It is indeed OK to discriminate "against" them, and to treat them differently legally, and to not invite them to dinner, and always has been.