To me this joke is funny because the father’s advice, well intentioned as it was, did not impart good manners, and the boy’s comments reveal his feeling about heavy girls.
To me, comments about a person that include the phrase “for his age (or other similar constructions) informs me about how a person thinks about others in the same category. If someone said, “She’s not loud, for a black woman,” or “He’s nice looking, for an Asian man,” or “She is thin, for a Latina”, we would be shocked and probably appalled. It’s no different when referring to someone’s age.
To me, Helen Mirren is beautiful- not for her age. The same is true for Jacqueline Bisset and Candice Bergin and millions of other mature women. The same is true for mature men. So the lesson is, next time you see me, you can say I look good, but forget about adding “for my age.” Ageism isn’t pretty at any age.