Judging someone on appearance is not OK. Doesn't matter if its height, weight, skin tone, or anything else. You shouldn't judge a person by their exterior because you don't know their story. There are hundreds of reasons someone might be overweight and most off them are out of a person's control.
You should start with a defined character. Compare someone old and young and how they can both be useful and smart no matter the age, argue the opposite as well.
So lets say, Jerry and his Grandad Patrick both do art. They both managed to land their art in galleries and are very successful. They both do commisions full time and can sustain themselves without needed another job. Although Jerry and Patrick are both highly skilled artists, it took Patrick 40 extra years to completely cultivate his talent.
^This is an example of the argument you are trying to write.
Think woodworking, both a 20 year old and a 60 year old can do it. Same with cleaning.
