You can't assume anything about someone just by looking at them. They might look young, healthy, and happy, but looks can often be deceiving. You would think that people would internalize the message of "don't judge a book by its cover" and carry it into adulthood, but that's just not the case.
I can't tell you guys how many videos I've seen of people harassing strangers for parking in a handicapped parking spot because they just assumed they were lying. I swear, some people think that being in a wheelchair or walking with a cane are the only disabilities in the world. Just because someone looks like they're not disabled doesn't give you free rein to yell at them. The same goes for demanding that people move from public transit seats reserved for the elderly, disabled, or pregnant. One teenage girl with a prosthetic leg had to deal with an ignorant person like this, and it's so upsetting to read about.